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Practice › LeetCode › LeetCode 34xx ›

LeetCode 3430 - Maximum and Minimum Sums of at Most Size K Subarrays

LeetCode Problem 3430 Difficulty: 🔴 Hard Topics: Array, Math, Stack, Monotonic Stack Solution Problem Understanding We are remains efficient even at the maximum constraint limit.

Practice › LeetCode › LeetCode 38xx ›

LeetCode 3878 - Count Good Subarrays

LeetCode Problem 3878 Difficulty: 🔴 Hard Topics: Array, Stack, Bit Manipulation, Monotonic Stack Solution LeetCode 3878 - Count Good Subarrays Problem Understanding We are given an integer array nums , and we must count how many subarrays are good . A subarray is considered good if the bitwise OR of all elements in that subarray is equal to at least one element that appears inside the same subarray. Suppose a...

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LeetCode 3834 - Merge Adjacent Equal Elements

LeetCode Problem 3834 Difficulty: 🟡 Medium Topics: Array, Stack, Simulation Solution Problem Understanding The problem asks us to repeatedly merge adjacent equal elements in an integer array nums until no further merges are possible. Specifically, if two consecutive elements are equal, we replace them with their sum. The key points are that we always pick the leftmost pair for merging, and after each merge, the array size decreases by one....

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LeetCode 3749 - Evaluate Valid Expressions

LeetCode Problem 3749 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Divide and Conquer, Stack Solution Problem Understanding The problem presents a simplified version of evaluating nested mathematical expressions in string format. The input expression is either a single integer literal, which may be negative, or a nested operation of the form op(a,b) where op is one of the four basic arithmetic operations: add , sub , mul , or...

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LeetCode 3746 - Minimum String Length After Balanced Removals

LeetCode Problem 3746 Difficulty: 🟡 Medium Topics: String, Stack, Counting Solution Before I write the full guide, I want to verify one detail because this appears to be a very new LeetCode problem and correctness matters here. The examples strongly suggest a key simplification: the answer may reduce to the absolute difference between the total number of 'a' and 'b' characters, because any balanced substring has net contribution 0 ,...

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LeetCode 3816 - Lexicographically Smallest String After Deleting Duplicate Characters

LeetCode Problem 3816 Difficulty: 🔴 Hard Topics: Hash Table, String, Stack, Greedy, Monotonic Stack Solution LeetCode 3816 - Lexicographically Smallest String After Deleting Duplicate Characters Problem Understanding We are given a string s consisting only of lowercase English letters. The allowed operation is very specific: if a character currently appears at least twice in the string, we may delete exactly one occurrence of that character. We may repeat this operation...

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LeetCode 3703 - Remove K-Balanced Substrings

LeetCode Problem 3703 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution LeetCode 3703 - Remove K-Balanced Substrings Problem Understanding We are given a string s consisting only of '(' and ')' , along with an integer k . A substring is considered k-balanced if it is exactly: "(" repeated k times + ")" repeated k times For example: k = 1 → "()" k = 2 → "(())" k =...

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LeetCode 3555 - Smallest Subarray to Sort in Every Sliding Window

LeetCode Problem 3555 Difficulty: 🟡 Medium Topics: Array, Two Pointers, Stack, Greedy, Sorting, Monotonic Stack Solution Problem Understanding The problem asks us to examine every contiguous subarray (window) of length k in the array nums and determine the minimum continuous segment within that window that must be sorted to make the entire window non-decreasing. In other words, for each sliding window, we want the smallest subarray that, if sorted, results...

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LeetCode 3412 - Find Mirror Score of a String

LeetCode Problem 3412 Difficulty: 🟡 Medium Topics: Hash Table, String, Stack, Simulation Solution LeetCode 3412 - Find Mirror Score of a String Problem Understanding We are given a lowercase English string s . Every letter has a unique "mirror" letter obtained by reversing the alphabet. For example: 'a' ↔ 'z' 'b' ↔ 'y' 'c' ↔ 'x' and so on. We process the string from left to right. For each position...

Practice › LeetCode › LeetCode 36xx ›

LeetCode 3676 - Count Bowl Subarrays

LeetCode Problem 3676 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack Solution Problem Understanding The problem asks us to count how many subarrays in an array of distinct integers satisfy a specific “bowl” condition. A subarray nums[l...r] is considered a bowl if it has length at least 3 and the smaller of its two endpoints is strictly greater than the maximum value of all elements strictly inside the subarray. In...

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LeetCode 3638 - Maximum Balanced Shipments

LeetCode Problem 3638 Difficulty: 🟡 Medium Topics: Array, Dynamic Programming, Stack, Greedy, Monotonic Stack Solution Problem Understanding We are given an array weight representing parcels arranged in a line. We must partition some prefix-suffix segments of this array into a collection of non-overlapping contiguous subarrays called shipments. Each parcel can belong to at most one shipment, and it is also allowed to leave some parcels unused. A shipment is considered...

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LeetCode 3561 - Resulting String After Adjacent Removals

LeetCode Problem 3561 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution Problem Understanding The problem asks us to repeatedly remove pairs of adjacent characters in a string that are consecutive in the alphabet, considering the alphabet as circular. "Consecutive in the alphabet" means two characters where the difference in their positions in the alphabet is 1, either forwards or backwards, and 'a' and 'z' are also considered consecutive. The operation...

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LeetCode 3542 - Minimum Operations to Convert All Elements to Zero

LeetCode Problem 3542 Difficulty: 🟡 Medium Topics: Array, Hash Table, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem requires us to reduce all elements of a given non-negative integer array nums to zero using the fewest number of operations. Each operation allows selecting a contiguous subarray and setting all occurrences of the minimum non-negative integer in that subarray to zero. In essence, we can remove the "smallest" numbers in...

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LeetCode 3523 - Make Array Non-decreasing

LeetCode Problem 3523 Difficulty: 🟡 Medium Topics: Array, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem asks us to transform an integer array nums into a non-decreasing array by performing zero or more operations, where an operation consists of selecting a contiguous subarray and replacing it with its maximum value . The goal is to maximize the size of the resulting array after all operations. In other words, we...

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LeetCode 3420 - Count Non-Decreasing Subarrays After K Operations

LeetCode Problem 3420 Difficulty: 🔴 Hard Topics: Array, Stack, Segment Tree, Queue, Sliding Window, Monotonic Stack, Monotonic Queue Solution Problem Understanding You are given an array nums and a budget of at most k increment operations. For any chosen subarray, you may repeatedly pick an element inside that subarray and increase it by 1 . The goal is to determine whether that subarray can be transformed into a non-decreasing array...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1963 - Minimum Number of Swaps to Make the String Balanced

LeetCode Problem 1963 Difficulty: 🟡 Medium Topics: Two Pointers, String, Stack, Greedy Solution Problem Understanding The problem asks us to transform a given string of brackets into a balanced string with the minimum number of swaps. The input string s consists of n characters, where n is even, and there are exactly n / 2 opening brackets '[' and n / 2 closing brackets ']' . A string is balanced...

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LeetCode 2434 - Using a Robot to Print the Lexicographically Smallest String

LeetCode Problem 2434 Difficulty: 🟡 Medium Topics: Hash Table, String, Stack, Greedy Solution Problem Understanding In this problem, we are given a string s and a robot that manages another temporary string t . Initially, t is empty. The robot repeatedly performs one of two allowed operations: First, it may remove the first character from s and append it to the end of t . Second, it may remove the...

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LeetCode 2865 - Beautiful Towers I

LeetCode Problem 2865 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack Solution Problem Understanding The problem asks us to transform an array of tower heights into a mountain-shaped arrangement while removing as few bricks as necessary. A mountain-shaped arrangement is defined as one where tower heights first non-decrease up to a peak (or consecutive peaks of the same height) and then non-increase after the peak. We are not allowed to...

Practice › LeetCode › LeetCode 19xx ›

LeetCode 1910 - Remove All Occurrences of a Substring

LeetCode Problem 1910 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution Problem Understanding The problem asks us to repeatedly remove occurrences of a substring part from a string s . The important detail is that on every operation, we must remove the leftmost occurrence of part . We continue performing removals until part no longer appears anywhere inside s . In simpler terms, we scan the string, remove one matching...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2216 - Minimum Deletions to Make Array Beautiful

LeetCode Problem 2216 Difficulty: 🟡 Medium Topics: Array, Stack, Greedy Solution Problem Understanding The problem asks us to transform a given integer array nums into a beautiful array using the minimum number of deletions. A beautiful array satisfies two conditions: its length is even, and no two consecutive elements at even indices are equal ( nums[i] != nums[i + 1] for all i % 2 == 0 ). Deletions are...

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LeetCode 1950 - Maximum of Minimum Values in All Subarrays

LeetCode Problem 1950 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack Solution Problem Understanding This problem asks us to compute, for every possible subarray size, the best possible minimum value among all subarrays of that size. Given an array nums of length n , we must evaluate every window size from 1 to n . For each size k , we consider all contiguous subarrays of length k . Every...

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LeetCode 2816 - Double a Number Represented as a Linked List

LeetCode Problem 2816 Difficulty: 🟡 Medium Topics: Linked List, Math, Stack Solution Problem Understanding This problem asks us to double a number represented as a singly linked list. Each node of the list contains a single digit, and the digits are stored in the order from most significant to least significant. For example, the list [1,8,9] represents the number 189. The task is to compute 2 * number and return...

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LeetCode 2130 - Maximum Twin Sum of a Linked List

LeetCode Problem 2130 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers, Stack Solution Problem Understanding The problem gives us a singly linked list with an even number of nodes. For every node at index i , there is a corresponding twin node at index n - 1 - i , where n is the total number of nodes in the list. We define the twin sum as: node[i].val + node[n...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2751 - Robot Collisions

LeetCode Problem 2751 Difficulty: 🔴 Hard Topics: Array, Stack, Sorting, Simulation Solution Problem Understanding This problem describes a simulation of robots moving on a one-dimensional line, each with a given starting position, health, and direction. Every robot moves at the same speed, and collisions occur when two robots meet at the same position. The key rules are: if two robots collide, the one with lower health is removed, the one...

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LeetCode 3205 - Maximum Array Hopping Score I

LeetCode Problem 3205 Difficulty: 🟡 Medium Topics: Array, Dynamic Programming, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem asks us to calculate the maximum score achievable by hopping through an array from the first element to the last. You start at index 0 , and at each step, you can jump to any subsequent index j > i . When you jump, you accumulate a score of (j -...

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LeetCode 3359 - Find Sorted Submatrices With Maximum Element at Most K

LeetCode Problem 3359 Difficulty: 🔴 Hard Topics: Array, Stack, Matrix, Monotonic Stack Solution Problem Understanding We are given a matrix grid with m rows and n columns, along with an integer k . The task is to count how many rectangular submatrices satisfy two conditions simultaneously: Every value inside the submatrix must be at most k . Every row inside the submatrix must be sorted in non-increasing order from left...

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LeetCode 2297 - Jump Game VIII

LeetCode Problem 2297 Difficulty: 🟡 Medium Topics: Array, Dynamic Programming, Stack, Graph Theory, Monotonic Stack, Shortest Path Solution Problem Understanding This problem asks us to compute the minimum total cost required to move from index 0 to index n - 1 in an array. The movement rules are unusual because whether a jump is valid depends not only on the values at the start and destination indices, but also on...

Practice › LeetCode › LeetCode 25xx ›

LeetCode 2524 - Maximum Frequency Score of a Subarray

LeetCode Problem 2524 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, Stack, Sliding Window Solution Problem Understanding The problem asks us to compute the maximum frequency score among all contiguous subarrays of length k from a given integer array nums . A frequency score is defined as the sum of each distinct element raised to the power of its frequency within the subarray, taken modulo 10^9 + 7 . For...

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LeetCode 3170 - Lexicographically Minimum String After Removing Stars

LeetCode Problem 3170 Difficulty: 🟡 Medium Topics: Hash Table, String, Stack, Greedy, Heap (Priority Queue) Solution Problem Understanding The problem asks us to transform a string s that may contain the '*' character into a lexicographically smallest string by repeatedly applying a deletion operation. Specifically, for each '*' in the string, we must remove it along with the smallest non- '*' character to its left . If there are multiple...

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LeetCode 2000 - Reverse Prefix of Word

LeetCode Problem 2000 Difficulty: 🟢 Easy Topics: Two Pointers, String, Stack Solution Problem Understanding The problem gives us a string word and a character ch . We must locate the first occurrence of ch inside the string. Once we find it, we reverse the substring starting at index 0 and ending at that occurrence, inclusive. After reversing that prefix, the remainder of the string stays exactly the same. For example,...

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LeetCode 2355 - Maximum Number of Books You Can Take

LeetCode Problem 2355 Difficulty: 🔴 Hard Topics: Array, Dynamic Programming, Stack, Monotonic Stack Solution That is a long, multi-section technical reference document. I can provide the full guide, but it will be quite extensive. Before I start, do you want the complete solution guide for LeetCode 2355 - Maximum Number of Books You Can Take in one single response , including all required sections (Problem Understanding, Approaches, Algorithm Walkthrough, Python...

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LeetCode 2019 - The Score of Students Solving Math Expression

LeetCode Problem 2019 Difficulty: 🔴 Hard Topics: Array, Hash Table, Math, String, Dynamic Programming, Stack, Memoization Solution LeetCode 2019, The Score of Students Solving Math Expression Problem Understanding The problem gives us a mathematical expression string s containing only: Single digit numbers 0-9 Addition operators + Multiplication operators * The expression is guaranteed to be valid. For example: 3+5*2 7+3*1*2 Students were supposed to evaluate the expression using the correct...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2281 - Sum of Total Strength of Wizards

LeetCode Problem 2281 Difficulty: 🔴 Hard Topics: Array, Stack, Monotonic Stack, Prefix Sum Solution Problem Understanding The problem asks us to compute the sum of total strengths across every possible contiguous subarray of the input array strength . For any chosen subarray, its total strength is defined as: $$\text{minimum value in subarray} \times \text{sum of values in subarray}$$ We must evaluate this quantity for all non-empty contiguous subarrays , then...

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LeetCode 2211 - Count Collisions on a Road

LeetCode Problem 2211 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution LeetCode 2211 - Count Collisions on a Road Problem Understanding The problem describes a one dimensional road containing cars positioned from left to right. Every car has one of three possible states: 'L' , meaning the car moves left 'R' , meaning the car moves right 'S' , meaning the car stays stationary All moving cars travel at the...

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LeetCode 2282 - Number of People That Can Be Seen in a Grid

LeetCode Problem 2282 Difficulty: 🟡 Medium Topics: Array, Stack, Matrix, Monotonic Stack Solution Problem Understanding The problem asks us to determine, for each person in a 2D grid of heights, how many other people they can see according to a specific line-of-sight rule. Each person at position (i, j) can see another person either directly to the right in the same row or directly below in the same column. The...

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LeetCode 2645 - Minimum Additions to Make Valid String

LeetCode Problem 2645 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Stack, Greedy Solution Problem Understanding This problem asks us to determine the minimum number of insertions needed to transform a given string word into a valid string . A valid string is defined as one that can be formed by concatenating the sequence "abc" one or more times. In other words, every valid string follows the repeating pattern: abcabcabc... We...

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LeetCode 2940 - Find Building Where Alice and Bob Can Meet

LeetCode Problem 2940 Difficulty: 🔴 Hard Topics: Array, Binary Search, Stack, Binary Indexed Tree, Segment Tree, Heap (Priority Queue), Monotonic Stack Solution Problem Understanding You are given an array heights , where each index represents a building and the value represents that building's height. A person standing at building i can move only to the right, meaning to some building j where j > i , and only if the...

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LeetCode 2116 - Check if a Parentheses String Can Be Valid

LeetCode Problem 2116 Difficulty: 🟡 Medium Topics: String, Stack, Greedy Solution Problem Understanding This problem asks whether a given parentheses string can be transformed into a valid parentheses sequence under certain constraints. We are given two strings: s , containing only '(' and ')' locked , containing only '0' and '1' Both strings have the same length n . At every index: If locked[i] == '1' , then s[i] is...

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LeetCode 3221 - Maximum Array Hopping Score II

LeetCode Problem 3221 Difficulty: 🟡 Medium Topics: Array, Stack, Greedy, Monotonic Stack Solution LeetCode 3221 - Maximum Array Hopping Score II Problem Understanding The problem gives us an integer array nums , where each position represents a possible location we can stand on. We always begin at index 0 , and we must eventually reach the last index of the array. From any index i , we are allowed to...

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LeetCode 2375 - Construct Smallest Number From DI String

LeetCode Problem 2375 Difficulty: 🟡 Medium Topics: String, Backtracking, Stack, Greedy Solution Problem Understanding The problem asks us to construct the lexicographically smallest number from a string pattern consisting of 'I' and 'D' . Each character in the pattern defines a relationship between consecutive digits of the number: 'I' indicates that the next digit should be larger than the current, and 'D' indicates the next digit should be smaller ....

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LeetCode 2104 - Sum of Subarray Ranges

LeetCode Problem 2104 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack Solution LeetCode 2104 - Sum of Subarray Ranges Problem Understanding The problem gives us an integer array nums , and asks us to compute the sum of the ranges of every possible non-empty contiguous subarray. The range of a subarray is defined as: maximum element - minimum element For every possible subarray, we calculate its range, then add all...

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LeetCode 2696 - Minimum String Length After Removing Substrings

LeetCode Problem 2696 Difficulty: 🟢 Easy Topics: String, Stack, Simulation Solution Problem Understanding The problem asks us to determine the minimum possible length of a string after repeatedly removing the substrings "AB" or "CD" . We are given a string s consisting solely of uppercase English letters. Each operation allows us to remove any occurrence of "AB" or "CD" from the string, and after removal, the remaining parts of the...

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LeetCode 2487 - Remove Nodes From Linked List

LeetCode Problem 2487 Difficulty: 🟡 Medium Topics: Linked List, Stack, Recursion, Monotonic Stack Solution Problem Understanding The problem gives us the head of a singly linked list and asks us to remove every node that has a strictly greater value somewhere to its right. In other words, for each node, we must determine whether there exists another node later in the list whose value is larger. If such a node...

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LeetCode 2589 - Minimum Time to Complete All Tasks

LeetCode Problem 2589 Difficulty: 🔴 Hard Topics: Array, Binary Search, Stack, Greedy, Sorting Solution Problem Understanding This problem is asking us to determine the minimum amount of time that a computer must be turned on to execute a set of tasks, given that each task has a specified time window [starti, endi] and requires a total duration durationi that does not need to be continuous. In other words, the computer...

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LeetCode 2345 - Finding the Number of Visible Mountains

LeetCode Problem 2345 Difficulty: 🟡 Medium Topics: Array, Stack, Sorting, Monotonic Stack Solution LeetCode 2345 - Finding the Number of Visible Mountains Problem Understanding Each mountain is represented by a peak point (x, y) . Because the mountain is a right-angled isosceles triangle with slopes +1 and -1 , its shape is completely determined by its peak. For a mountain with peak (x, y) : The left boundary touches the...

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LeetCode 2863 - Maximum Length of Semi-Decreasing Subarrays

LeetCode Problem 2863 Difficulty: 🟡 Medium Topics: Array, Stack, Sorting, Monotonic Stack Solution LeetCode 2863 - Maximum Length of Semi-Decreasing Subarrays Problem Understanding The problem gives us an integer array nums and asks for the length of the longest contiguous subarray whose first element is strictly greater than its last element. A subarray is any contiguous segment of the array. For a subarray nums[i...j] to be considered semi-decreasing, the following...

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LeetCode 2813 - Maximum Elegance of a K-Length Subsequence

LeetCode Problem 2813 Difficulty: 🔴 Hard Topics: Array, Hash Table, Stack, Greedy, Sorting, Heap (Priority Queue) Solution Problem Understanding The problem presents a 0-indexed 2D array items of length n , where each element represents an item with two values: a profit and a category. You are asked to select exactly k items to form a subsequence . A subsequence preserves the original order of items but can skip elements....

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LeetCode 1896 - Minimum Cost to Change the Final Value of Expression

LeetCode Problem 1896 Difficulty: 🔴 Hard Topics: Math, String, Dynamic Programming, Stack Solution Problem Understanding This problem gives us a boolean expression containing only: '0' and '1' binary operators '&' and '|' parentheses The expression is guaranteed to be valid, which means every operator has valid operands and every parenthesis is properly matched. The important detail is that the expression is not evaluated using normal operator precedence. Instead: Parentheses are...

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LeetCode 2764 - Is Array a Preorder of Some ‌Binary Tree

LeetCode Problem 2764 Difficulty: 🟡 Medium Topics: Stack, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem asks us to determine whether a given list of nodes represents the preorder traversal of a binary tree. Each node is represented as a pair [id, parentId] , where id is the unique identifier of the node, and parentId identifies its parent in the tree. A parentId of -1 indicates that the...

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LeetCode 1966 - Binary Searchable Numbers in an Unsorted Array

LeetCode Problem 1966 Difficulty: 🟡 Medium Topics: Array, Binary Search, Stack, Monotonic Stack Solution Problem Understanding This problem asks us to identify how many numbers in an unsorted array are still guaranteed to be found by a randomized binary-search-like process. The array nums contains unique integers. The search procedure repeatedly chooses a random pivot element from the current remaining sequence. Depending on whether the pivot is smaller or larger than...

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LeetCode 3174 - Clear Digits

LeetCode Problem 3174 Difficulty: 🟢 Easy Topics: String, Stack, Simulation Solution Problem Understanding In this problem, we are given a string s that contains lowercase English letters and digits. The task is to repeatedly remove digits according to a very specific rule. Whenever we encounter a digit, we must delete two characters: The digit itself The closest non-digit character to its left This process continues until all digits are removed....

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LeetCode 2945 - Find Maximum Non-decreasing Array Length

LeetCode Problem 2945 Difficulty: 🔴 Hard Topics: Array, Binary Search, Dynamic Programming, Stack, Queue, Monotonic Stack, Monotonic Queue Solution Problem Understanding This problem asks us to determine the maximum length of a non-decreasing array that can be obtained from a given integer array nums by performing a sequence of subarray sum operations . In each operation, we can choose any contiguous subarray, replace it with a single element equal to...

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LeetCode 2454 - Next Greater Element IV

LeetCode Problem 2454 Difficulty: 🔴 Hard Topics: Array, Binary Search, Stack, Sorting, Heap (Priority Queue), Monotonic Stack Solution Problem Understanding The problem asks us to compute, for every element in the array, its "second greater element" to the right. For an index i , we are interested in elements positioned after i . Among those elements, we only care about values strictly greater than nums[i] . The second greater element...

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LeetCode 2818 - Apply Operations to Maximize Score

LeetCode Problem 2818 Difficulty: 🔴 Hard Topics: Array, Math, Stack, Greedy, Sorting, Monotonic Stack, Number Theory Solution Problem Understanding The problem gives us an array nums and an integer k . We begin with a score of 1 , and we are allowed to perform at most k operations. In each operation, we choose a subarray that has not been chosen before. From that subarray, we select the element with...

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LeetCode 2334 - Subarray With Elements Greater Than Varying Threshold

LeetCode Problem 2334 Difficulty: 🔴 Hard Topics: Array, Stack, Union-Find, Monotonic Stack Solution Problem Understanding This problem asks us to find a contiguous subarray whose elements are all sufficiently large relative to the subarray length. More specifically, for a subarray of length k , every element inside that subarray must satisfy: $$\text{element} > \frac{\text{threshold}}{k}$$ We are allowed to return the size k of any valid subarray. If no such subarray...

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LeetCode 2296 - Design a Text Editor

LeetCode Problem 2296 Difficulty: 🔴 Hard Topics: Linked List, String, Stack, Design, Simulation, Doubly-Linked List Solution Problem Understanding This problem asks us to design a simplified text editor that supports four operations: Inserting text at the current cursor position Deleting characters to the left of the cursor Moving the cursor left Moving the cursor right The cursor behaves exactly like a cursor in a normal text editor. It always sits...

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LeetCode 2617 - Minimum Number of Visited Cells in a Grid

LeetCode Problem 2617 Difficulty: 🔴 Hard Topics: Array, Dynamic Programming, Stack, Breadth-First Search, Union-Find, Heap (Priority Queue), Matrix, Monotonic Stack Solution Problem Understanding The problem asks us to determine the minimum number of cells we need to visit in order to reach the bottom-right corner of a given m x n grid. Each cell (i, j) contains a number grid[i][j] representing the maximum number of steps we can move right...

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LeetCode 1944 - Number of Visible People in a Queue

LeetCode Problem 1944 Difficulty: 🔴 Hard Topics: Array, Stack, Monotonic Stack Solution Problem Understanding The problem gives a queue of n people standing from left to right, where each person has a distinct height. For each person i , we need to determine how many people to their right they can see. A person i can see a person j (with j > i ) if every person between them...

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LeetCode 2030 - Smallest K-Length Subsequence With Occurrences of a Letter

LeetCode Problem 2030 Difficulty: 🔴 Hard Topics: String, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem asks us to construct the lexicographically smallest subsequence of length k from the string s , while ensuring that a specific character, letter , appears at least repetition times in the resulting subsequence. A subsequence preserves the relative order of characters from the original string, but characters may be skipped. For example, from...

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LeetCode 3113 - Find the Number of Subarrays Where Boundary Elements Are Maximum

LeetCode Problem 3113 Difficulty: 🔴 Hard Topics: Array, Binary Search, Stack, Monotonic Stack Solution Problem Understanding We are given an array nums of positive integers. We need to count how many contiguous subarrays satisfy a very specific condition: The first element of the subarray and the last element of the subarray must both be equal to the maximum value inside that subarray. In other words, for a subarray nums[l:r+1] ,...

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LeetCode 1996 - The Number of Weak Characters in the Game

LeetCode Problem 1996 Difficulty: 🟡 Medium Topics: Array, Stack, Greedy, Sorting, Monotonic Stack Solution Problem Understanding In this problem, every character in the game has two attributes: attack defense The input is a 2D array called properties , where: properties[i] = [attacki, defensei] represents the stats of the i-th character. A character is considered weak if there exists another character with: strictly greater attack strictly greater defense Both conditions must...

Practice › LeetCode › LeetCode 28xx ›

LeetCode 2866 - Beautiful Towers II

LeetCode Problem 2866 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack Solution Problem Understanding The problem asks us to build a series of towers along a coordinate line, where each tower has a height constrained by a given maxHeights array. The resulting configuration must form a mountain array , meaning there is a single peak somewhere in the array, with tower heights non-decreasing up to the peak and non-increasing after...

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LeetCode 2832 - Maximal Range That Each Element Is Maximum in It

LeetCode Problem 2832 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack Solution LeetCode 2832 - Maximal Range That Each Element Is Maximum in It Problem Understanding We are given an array nums consisting of distinct integers. For every position i , we need to determine the maximum possible length of a contiguous subarray in which nums[i] is the largest element. More formally, for each index i , we want to...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2390 - Removing Stars From a String

LeetCode Problem 2390 Difficulty: 🟡 Medium Topics: String, Stack, Simulation Solution Problem Understanding This problem asks us to repeatedly process a string that contains lowercase English letters and star characters ( * ). Every star represents a removal operation. When we encounter a star, we must remove two things: The star itself. The closest non-star character immediately to its left. We continue this process until all stars have been removed....

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LeetCode 2197 - Replace Non-Coprime Numbers in Array

LeetCode Problem 2197 Difficulty: 🔴 Hard Topics: Array, Math, Stack, Number Theory Solution Problem Understanding The problem asks us to repeatedly replace adjacent non-coprime numbers in an array with their Least Common Multiple (LCM) until no more adjacent non-coprime pairs exist. Two numbers are non-coprime if their greatest common divisor (GCD) is greater than 1. The final array should be returned after all possible replacements. In other words, given an...

Practice › LeetCode › LeetCode 27xx ›

LeetCode 2736 - Maximum Sum Queries

LeetCode Problem 2736 Difficulty: 🔴 Hard Topics: Array, Binary Search, Stack, Binary Indexed Tree, Segment Tree, Sorting, Monotonic Stack Solution LeetCode 2736 - Maximum Sum Queries Problem Understanding We are given two arrays, nums1 and nums2 , both of length n . Each index j represents a point: $$(nums1[j], nums2[j])$$ and has an associated value: $$nums1[j] + nums2[j]$$ For every query [xi, yi] , we must find an index j...

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LeetCode 3229 - Minimum Operations to Make Array Equal to Target

LeetCode Problem 3229 Difficulty: 🔴 Hard Topics: Array, Dynamic Programming, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem gives us two arrays, nums and target , both of the same length. We are allowed to perform operations on nums until it becomes exactly equal to target . In one operation, we choose any contiguous subarray and either: Increment every element in that subarray by 1 Decrement every element in...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1856 - Maximum Subarray Min-Product

LeetCode Problem 1856 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack, Prefix Sum Solution Problem Understanding The problem asks us to find the maximum min-product of any contiguous subarray of a given integer array nums . The min-product of a subarray is defined as the minimum value in that subarray multiplied by the sum of all elements in that subarray . We are asked to return this maximum value modulo...

Practice › LeetCode › LeetCode 22xx ›

LeetCode 2289 - Steps to Make Array Non-decreasing

LeetCode Problem 2289 Difficulty: 🟡 Medium Topics: Array, Linked List, Dynamic Programming, Stack, Monotonic Stack, Simulation Solution Problem Understanding The problem asks us to determine how many steps it takes to make a given array nums non-decreasing by repeatedly removing elements that break the non-decreasing property. Specifically, for each step, any element nums[i] where nums[i - 1] > nums[i] is removed. This process continues until the array is entirely non-decreasing....

Practice › LeetCode › LeetCode 00xx ›

LeetCode 94 - Binary Tree Inorder Traversal

LeetCode Problem 94 Difficulty: 🟢 Easy Topics: Stack, Tree, Depth-First Search, Binary Tree Solution Binary Tree Inorder Traversal Problem Understanding This problem asks us to perform an inorder traversal on a binary tree and return the values of the visited nodes in the correct order. A binary tree is a hierarchical data structure where each node can have at most two children, a left child and a right child. The...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 232 - Implement Queue using Stacks

LeetCode Problem 232 Difficulty: 🟢 Easy Topics: Stack, Design, Queue Solution Problem Understanding The problem asks us to implement a queue using only stack operations. A queue follows the FIFO, First In First Out, principle. The first element inserted into the queue must be the first element removed. In contrast, a stack follows the LIFO, Last In First Out, principle, where the most recently inserted element is removed first. We...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 84 - Largest Rectangle in Histogram

LeetCode Problem 84 Difficulty: 🔴 Hard Topics: Array, Stack, Monotonic Stack Solution LeetCode 84 - Largest Rectangle in Histogram Problem Understanding This problem asks us to find the largest rectangular area that can be formed inside a histogram. A histogram is represented as an array of integers called heights , where each integer describes the height of a bar, and every bar has a width of exactly 1 . The...

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LeetCode 581 - Shortest Unsorted Continuous Subarray

LeetCode Problem 581 Difficulty: 🟡 Medium Topics: Array, Two Pointers, Stack, Greedy, Sorting, Monotonic Stack Solution Problem Understanding The problem gives an integer array nums and asks for the length of the shortest continuous subarray such that, if only that subarray is sorted in non-decreasing order, the entire array becomes sorted. In other words, the array is "almost sorted", except for some misplaced elements. We need to identify the smallest...

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LeetCode 20 - Valid Parentheses

LeetCode Problem 20 Difficulty: 🟢 Easy Topics: String, Stack Solution Problem Understanding The problem gives a string containing only six possible characters: ( , ) , { , } , [ and ] . Each opening bracket must eventually be matched with the correct closing bracket, and the order of matching matters. A string is considered valid only if every opening bracket is closed by the same type of bracket...

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LeetCode 1021 - Remove Outermost Parentheses

LeetCode Problem 1021 Difficulty: 🟢 Easy Topics: String, Stack Solution Unusual activity has been detected from your device. Try again later. (36c3723c-3fa3-4b63-a514-20d2635d5e47)

Practice › LeetCode › LeetCode 01xx ›

LeetCode 143 - Reorder List

LeetCode Problem 143 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers, Stack, Recursion Solution Problem Understanding The problem gives us the head of a singly linked list and asks us to reorder the nodes in a very specific alternating pattern. A normal linked list looks like this: L0 → L1 → L2 → ... → Ln We must transform it into: L0 → Ln → L1 → Ln-1 → L2...

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LeetCode 1172 - Dinner Plate Stacks

LeetCode Problem 1172 Difficulty: 🔴 Hard Topics: Hash Table, Stack, Design, Heap (Priority Queue) Solution Problem Understanding The problem asks us to design a data structure that behaves like an infinite row of stacks, where every stack has the same fixed capacity. Instead of working with a single stack, we maintain many stacks indexed from left to right starting at 0 . The operations are constrained by specific rules: When...

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LeetCode 1249 - Minimum Remove to Make Valid Parentheses

LeetCode Problem 1249 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem is asking us to process a string containing lowercase letters and parentheses, and remove the minimum number of parentheses so that the remaining string is valid. A valid string is defined by the usual rules of parentheses: every opening parenthesis '(' must have a corresponding closing parenthesis ')' , and parentheses must be properly nested. The...

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LeetCode 1081 - Smallest Subsequence of Distinct Characters

LeetCode Problem 1081 Difficulty: 🟡 Medium Topics: String, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem asks us to construct the lexicographically smallest subsequence of a string that contains every distinct character exactly once. A subsequence is formed by deleting zero or more characters without changing the relative order of the remaining characters. This is important because we are not allowed to rearrange characters arbitrarily. The final answer must...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1574 - Shortest Subarray to be Removed to Make Array Sorted

LeetCode Problem 1574 Difficulty: 🟡 Medium Topics: Array, Two Pointers, Binary Search, Stack, Monotonic Stack Solution Please provide the full problem statement or LeetCode number you want the detailed solution guide for. You mentioned LeetCode 1574 - Shortest Subarray to be Removed to Make Array Sorted , so I can generate the complete guide with all requested sections: Problem Understanding Approaches (Brute Force + Optimal) Comparison Table Algorithm Walkthrough Python...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 85 - Maximal Rectangle

LeetCode Problem 85 Difficulty: 🔴 Hard Topics: Array, Dynamic Programming, Stack, Matrix, Monotonic Stack Solution Problem Understanding The problem asks us to find the area of the largest rectangle containing only 1 s in a binary matrix. The matrix consists of characters '0' and '1' , where '1' represents a valid cell that may belong to a rectangle and '0' represents a blocked cell that cannot be included. The key...

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LeetCode 1597 - Build Binary Expression Tree From Infix Expression

LeetCode Problem 1597 Difficulty: 🔴 Hard Topics: String, Stack, Tree, Binary Tree Solution Problem Understanding The problem asks us to construct a binary expression tree from a valid infix arithmetic expression string. An infix expression is the standard mathematical notation where operators appear between operands, such as 3+4 or 2*(5-1) . The input string may contain: Single digit operands Operators: + , - , * , / Parentheses ( and...

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LeetCode 589 - N-ary Tree Preorder Traversal

LeetCode Problem 589 Difficulty: 🟢 Easy Topics: Stack, Tree, Depth-First Search Solution Problem Understanding The problem asks for a preorder traversal of an n-ary tree. In a preorder traversal, the order of visiting nodes is: first the root node, then recursively all the children from left to right. The input is given as the root node of an n-ary tree, where each node has a value and a list of...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 155 - Min Stack

LeetCode Problem 155 Difficulty: 🟡 Medium Topics: Stack, Design Solution Problem Understanding The problem asks us to design a custom stack data structure that behaves like a normal stack, while also supporting an additional operation called getMin() . This operation must return the minimum value currently stored in the stack, and it must do so in constant time, O(1) . A normal stack already supports these standard operations efficiently: push(val)...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 936 - Stamping The Sequence

LeetCode Problem 936 Difficulty: 🔴 Hard Topics: String, Stack, Greedy, Queue Solution Problem Understanding This problem asks us to reconstruct a target string by repeatedly applying a smaller string called stamp . We begin with a string s that has the same length as target , but every character is initially '?' . In one operation, we may place the stamp string onto any valid position in s , and...

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LeetCode 921 - Minimum Add to Make Parentheses Valid

LeetCode Problem 921 Difficulty: 🟡 Medium Topics: String, Stack, Greedy Solution Problem Understanding The problem asks us to determine the minimum number of parentheses insertions required to make a given string of parentheses valid. A string of parentheses is considered valid if every opening parenthesis '(' has a matching closing parenthesis ')' and the pairs are properly nested. The input s is a string consisting only of '(' and ')'...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 227 - Basic Calculator II

LeetCode Problem 227 Difficulty: 🟡 Medium Topics: Math, String, Stack Solution Problem Understanding The problem gives us a string representing a mathematical expression containing non-negative integers, spaces, and the four operators + , - , * , and / . Our task is to evaluate the expression and return the resulting integer value. The important detail is that multiplication and division must follow normal arithmetic precedence rules. This means *...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 739 - Daily Temperatures

LeetCode Problem 739 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack Solution Problem Understanding The problem gives an array called temperatures , where each element represents the temperature recorded on a specific day. For every day, we must determine how many days in the future we need to wait until a warmer temperature appears. If there is no future day with a higher temperature, the answer for that position should...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1047 - Remove All Adjacent Duplicates In String

LeetCode Problem 1047 Difficulty: 🟢 Easy Topics: String, Stack Solution Problem Understanding The problem gives us a string s containing only lowercase English letters. We repeatedly look for two adjacent characters that are equal, remove both of them, and continue this process until no such adjacent duplicate pair exists. The important detail is that removals can create new adjacent duplicates that were not adjacent before. For example, in the string...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 716 - Max Stack

LeetCode Problem 716 Difficulty: 🔴 Hard Topics: Linked List, Stack, Design, Doubly-Linked List, Ordered Set Solution Problem Understanding The problem asks us to design a stack data structure, MaxStack , which behaves like a normal stack with additional operations for efficiently accessing and removing the maximum element. Specifically, in addition to the standard stack operations push , pop , and top , we need to implement peekMax , which returns...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1441 - Build an Array With Stack Operations

LeetCode Problem 1441 Difficulty: 🟡 Medium Topics: Array, Stack, Simulation Solution Problem Understanding The problem gives us a strictly increasing array called target and an integer n . We are also given access to a stream of integers starting from 1 and ending at n . The numbers arrive in order, and once a number is skipped, we cannot go back to it. We start with an empty stack and...

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LeetCode 1003 - Check If Word Is Valid After Substitutions

LeetCode Problem 1003 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem asks us to determine whether a given string can be constructed by repeatedly inserting the substring "abc" into an initially empty string. The operation is very specific. At any point, we may take the current string and insert "abc" at any position. Because insertion can happen anywhere, valid strings may contain nested or interleaved patterns that...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 255 - Verify Preorder Sequence in Binary Search Tree

LeetCode Problem 255 Difficulty: 🟡 Medium Topics: Array, Stack, Tree, Binary Search Tree, Recursion, Monotonic Stack, Binary Tree Solution Problem Understanding The problem gives an array of unique integers called preorder . This array is supposed to represent the preorder traversal of a binary search tree, and we must determine whether such a BST could actually exist. In a preorder traversal, nodes are visited in this order: Visit the current...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 880 - Decoded String at Index

LeetCode Problem 880 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem gives us an encoded string s that represents a potentially enormous decoded string. The decoding process works incrementally from left to right. When we encounter a letter, we append it directly to the decoded tape. When we encounter a digit d , we repeat the entire current decoded tape d times in total. In other words,...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1544 - Make The String Great

LeetCode Problem 1544 Difficulty: 🟢 Easy Topics: String, Stack Solution Problem Understanding The problem asks us to process a string s consisting of lowercase and uppercase English letters, removing pairs of adjacent letters that are the same character but differ in case. Specifically, any pair where one character is lowercase and the adjacent character is its uppercase counterpart (or vice versa) is considered "bad." We must remove such pairs iteratively...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 678 - Valid Parenthesis String

LeetCode Problem 678 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Stack, Greedy Solution Problem Understanding This problem is asking whether a string containing the characters '(' , ')' , and '*' can be considered a valid parenthesis string. In essence, '(' must be matched by a ')' in the correct order, and '*' can act as a flexible placeholder that behaves as '(' , ')' , or an empty string....

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LeetCode 145 - Binary Tree Postorder Traversal

LeetCode Problem 145 Difficulty: 🟢 Easy Topics: Stack, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem asks us to return the postorder traversal of a binary tree. A binary tree is made of nodes, where each node contains: A value A pointer to a left child A pointer to a right child In postorder traversal, we must visit nodes in this exact order: Traverse the left subtree Traverse...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 726 - Number of Atoms

LeetCode Problem 726 Difficulty: 🔴 Hard Topics: Hash Table, String, Stack, Sorting Solution Problem Understanding This problem asks us to parse a chemical formula represented as a string and return a canonical string that counts the number of each atom in the formula. Each atom starts with an uppercase letter and may have lowercase letters following it. A number following an atom represents how many of that atom are present,...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 488 - Zuma Game

LeetCode Problem 488 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Stack, Breadth-First Search, Memoization Solution Problem Understanding The problem models a recursive elimination game played on a row of colored balls. The board is represented as a string where each character corresponds to a colored ball. The available colors are 'R' , 'Y' , 'B' , 'G' , and 'W' . You are also given another string called hand ,...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 844 - Backspace String Compare

LeetCode Problem 844 Difficulty: 🟢 Easy Topics: Two Pointers, String, Stack, Simulation Solution LeetCode 844 - Backspace String Compare Problem Understanding This problem asks us to determine whether two strings become identical after processing backspace operations. Each string represents characters typed into a text editor, where the '#' character means "delete the previous character", similar to pressing the backspace key on a keyboard. We are given two input strings, s...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1700 - Number of Students Unable to Eat Lunch

LeetCode Problem 1700 Difficulty: 🟢 Easy Topics: Array, Stack, Queue, Simulation Solution Problem Understanding This problem simulates a school cafeteria where students line up to take sandwiches from a stack. Each student has a preference for either a circular sandwich ( 0 ) or a square sandwich ( 1 ). The sandwiches are arranged in a stack where the first element of the sandwiches array represents the top of the...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 536 - Construct Binary Tree from String

LeetCode Problem 536 Difficulty: 🟡 Medium Topics: String, Stack, Tree, Depth-First Search, Binary Tree Solution Problem Understanding This problem asks us to reconstruct a binary tree from a specially formatted string representation. The input string contains integer values and parentheses. Every integer represents a tree node, and parentheses represent child subtrees. The structure follows a recursive pattern: A node value appears first. If the node has children, they are enclosed...

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LeetCode 1209 - Remove All Adjacent Duplicates in String II

LeetCode Problem 1209 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem asks us to repeatedly remove groups of exactly k adjacent identical characters from a string until no more such groups exist. A removal operation works as follows: Find k consecutive characters that are all the same. Remove those characters from the string. Concatenate the remaining left and right parts together. Continue checking again because new adjacent...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1008 - Construct Binary Search Tree from Preorder Traversal

LeetCode Problem 1008 Difficulty: 🟡 Medium Topics: Array, Stack, Tree, Binary Search Tree, Monotonic Stack, Binary Tree Solution Let's dive deep and construct a thorough technical guide for LeetCode 1008, following your formatting rules. Problem Understanding The problem asks us to construct a binary search tree (BST) from a given preorder traversal array. A preorder traversal is defined as visiting the root node first, then recursively visiting the left subtree,...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 496 - Next Greater Element I

LeetCode Problem 496 Difficulty: 🟢 Easy Topics: Array, Hash Table, Stack, Monotonic Stack Solution Problem Understanding The problem is asking us to find the next greater element for each element in nums1 within another array nums2 . Formally, for each element in nums1 , we need to locate its position in nums2 and then find the first element to its right in nums2 that is greater than itself . If...

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LeetCode 445 - Add Two Numbers II

LeetCode Problem 445 Difficulty: 🟡 Medium Topics: Linked List, Math, Stack Solution Problem Understanding The problem gives us two non-empty singly linked lists where each node stores a single digit of a non-negative integer. Unlike the classic "Add Two Numbers" problem, the digits are stored in forward order, meaning the most significant digit appears first. For example, the linked list: 7 -> 2 -> 4 -> 3 represents the integer...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 636 - Exclusive Time of Functions

LeetCode Problem 636 Difficulty: 🟡 Medium Topics: Array, Stack Solution Problem Understanding This problem models how functions execute on a single-threaded CPU. Since the CPU is single-threaded, only one function can actively execute at any given moment. However, functions may call other functions, including recursive calls to themselves. Whenever one function calls another, the currently running function pauses, and the newly called function begins executing immediately. The input consists of:...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 173 - Binary Search Tree Iterator

LeetCode Problem 173 Difficulty: 🟡 Medium Topics: Stack, Tree, Design, Binary Search Tree, Binary Tree, Iterator Solution It looks like your prompt accidentally concatenated two problems. I will answer the second one, LeetCode 173 - Binary Search Tree Iterator , following your required format. Problem Understanding This problem asks us to design an iterator for a Binary Search Tree (BST) that returns values in in-order traversal order . In a...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1628 - Design an Expression Tree With Evaluate Function

LeetCode Problem 1628 Difficulty: 🟡 Medium Topics: Array, Math, Stack, Tree, Design, Binary Tree Solution Problem Understanding The problem requires constructing a binary expression tree from a postfix arithmetic expression and implementing a method to evaluate the tree. The input is an array of strings postfix representing numbers and operators ( + , - , * , / ) in postfix notation , meaning operands appear before their operators. The...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 32 - Longest Valid Parentheses

LeetCode Problem 32 Difficulty: 🔴 Hard Topics: String, Dynamic Programming, Stack Solution Problem Understanding The problem asks us to find the length of the longest contiguous substring that forms a valid parentheses sequence. A valid parentheses sequence is one where every opening parenthesis '(' has a corresponding closing parenthesis ')' , and the pairs are correctly nested. For example, the string "()()" is valid because each opening parenthesis is matched...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 772 - Basic Calculator III

LeetCode Problem 772 Difficulty: 🔴 Hard Topics: Math, String, Stack, Recursion Solution LeetCode 772 - Basic Calculator III Problem Understanding This problem asks us to evaluate a mathematical expression represented as a string. The expression may contain: Non-negative integers Addition ( + ) Subtraction ( - ) Multiplication ( * ) Division ( / ) Parentheses ( ( and ) ) The goal is to compute the final integer result...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 503 - Next Greater Element II

LeetCode Problem 503 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack Solution Problem Understanding The problem gives us a circular array nums , and for every element, we must find the next greater element . The phrase next greater element means the first value encountered while moving forward in the array that is strictly larger than the current number. The key complication is that the array is circular . Normally,...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 71 - Simplify Path

LeetCode Problem 71 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem gives us an absolute Unix-style file path and asks us to convert it into its canonical, simplified form. An absolute path always starts from the root directory, represented by / . The input path may contain unnecessary components such as repeated slashes, references to the current directory, or references to parent directories. Our job is to...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1541 - Minimum Insertions to Balance a Parentheses String

LeetCode Problem 1541 Difficulty: 🟡 Medium Topics: String, Stack, Greedy Solution Problem Understanding This problem asks us to transform a string of parentheses into a special kind of balanced parentheses string using the minimum number of insertions. Unlike the standard parentheses balancing problem, this problem defines balance differently. Every opening parenthesis '(' must be matched with exactly two consecutive closing parentheses '))' . That means: One '(' requires two ')'...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 591 - Tag Validator

LeetCode Problem 591 Difficulty: 🔴 Hard Topics: String, Stack Solution Problem Understanding This problem asks us to validate whether a given string represents a correctly structured code snippet according to a custom XML-like syntax. At first glance, it resembles parsing HTML or XML tags, but the validation rules are stricter and more specialized. The input is a single string code , which may contain opening tags, closing tags, CDATA sections,...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 484 - Find Permutation

LeetCode Problem 484 Difficulty: 🟡 Medium Topics: Array, String, Stack, Greedy Solution Problem Understanding The problem gives us a string s consisting only of the characters 'I' and 'D' . This string describes the relationship between adjacent numbers in a hidden permutation. If s[i] == 'I' , then the permutation must satisfy: perm[i] < perm[i + 1] If s[i] == 'D' , then the permutation must satisfy: perm[i] > perm[i...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1762 - Buildings With an Ocean View

LeetCode Problem 1762 Difficulty: 🟡 Medium Topics: Array, Stack, Monotonic Stack Solution LeetCode 1762, Buildings With an Ocean View Problem Understanding This problem gives us an array called heights , where each element represents the height of a building. The buildings are arranged in a straight line from left to right, and the ocean is located to the right side of the last building. A building has an ocean view...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 901 - Online Stock Span

LeetCode Problem 901 Difficulty: 🟡 Medium Topics: Stack, Design, Monotonic Stack, Data Stream Solution Problem Understanding The problem asks us to design a class that processes stock prices one day at a time and, for every new price, returns the stock span for that day. The stock span is defined as the number of consecutive days ending today where the stock price was less than or equal to today's price....

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1190 - Reverse Substrings Between Each Pair of Parentheses

LeetCode Problem 1190 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem gives us a string s containing lowercase English letters and balanced parentheses. Our task is to repeatedly reverse the substrings enclosed inside every matching pair of parentheses, starting from the innermost pair first. After all reversals are complete, we must return the final string without any parentheses. The important detail is that nested parentheses matter. When...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1586 - Binary Search Tree Iterator II

LeetCode Problem 1586 Difficulty: 🟡 Medium Topics: Stack, Tree, Design, Binary Search Tree, Binary Tree, Iterator Solution Problem Understanding The problem requires implementing a BSTIterator class that allows forward and backward traversal over the in-order sequence of a Binary Search Tree (BST). In-order traversal of a BST visits nodes in ascending order. The iterator must support four operations: next() , prev() , hasNext() , and hasPrev() . Initially, the iterator...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1472 - Design Browser History

LeetCode Problem 1472 Difficulty: 🟡 Medium Topics: Array, Linked List, Stack, Design, Doubly-Linked List, Data Stream Solution Problem Understanding The problem is asking us to simulate a single-tab browser with a history mechanism. You start on a homepage , and from there, you can visit new URLs, back track a certain number of steps, or forward a certain number of steps. The inputs consist of method calls: BrowserHistory(homepage) initializes the...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 42 - Trapping Rain Water

LeetCode Problem 42 Difficulty: 🔴 Hard Topics: Array, Two Pointers, Dynamic Programming, Stack, Monotonic Stack Solution Problem Understanding The problem gives an array called height , where each element represents the height of a vertical bar in an elevation map. Every bar has width 1 . After rainfall, water may become trapped between taller bars. The task is to compute the total amount of water that can be trapped. For...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 341 - Flatten Nested List Iterator

LeetCode Problem 341 Difficulty: 🟡 Medium Topics: Stack, Tree, Depth-First Search, Design, Queue, Iterator Solution Problem Understanding This problem asks us to design an iterator that can traverse a deeply nested list structure as if it were a flat list of integers. The input is not a normal array. Each element inside nestedList can be one of two things: A single integer Another list containing more NestedInteger objects Those nested...

Practice › LeetCode › LeetCode 05xx ›

LeetCode 590 - N-ary Tree Postorder Traversal

LeetCode Problem 590 Difficulty: 🟢 Easy Topics: Stack, Tree, Depth-First Search Solution Problem Understanding This problem asks us to perform a postorder traversal on an n-ary tree. In a binary tree, each node has at most two children. In an n-ary tree, each node can have any number of children. Every node contains a value and a list of child nodes. A postorder traversal means: Visit all children first Then...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1063 - Number of Valid Subarrays

LeetCode Problem 1063 Difficulty: 🔴 Hard Topics: Array, Stack, Monotonic Stack Solution Problem Understanding The problem is asking us to count the number of valid subarrays in a given integer array nums . A subarray is valid if its leftmost element is not larger than any other element in the subarray. In other words, for a subarray starting at index i and ending at index j , the condition nums[i]...

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LeetCode 1793 - Maximum Score of a Good Subarray

LeetCode Problem 1793 Difficulty: 🔴 Hard Topics: Array, Two Pointers, Binary Search, Stack, Monotonic Stack Solution Problem Understanding In this problem, we are given an integer array nums and a specific index k . We must find a contiguous subarray that contains index k , and among all such subarrays, maximize the following score: $$\text{score} = \min(\text{subarray}) \times \text{length of subarray}$$ The minimum element inside the chosen subarray determines the...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 856 - Score of Parentheses

LeetCode Problem 856 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem is asking us to compute a score for a string of balanced parentheses according to a set of rules. A balanced parentheses string is one in which every opening parenthesis '(' has a corresponding closing parenthesis ')' and the pairs are properly nested. The rules for scoring are as follows: A single pair "()" scores 1...

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LeetCode 1087 - Brace Expansion

LeetCode Problem 1087 Difficulty: 🟡 Medium Topics: String, Backtracking, Stack, Breadth-First Search, Sorting Solution Problem Understanding In this problem, we are given a string that represents multiple possible words. Each position in the word may contain either a single fixed character or a set of alternative characters enclosed in curly braces. For example: a{b,c}d means: The first character is always 'a' The second character can be either 'b' or 'c'...

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LeetCode 1124 - Longest Well-Performing Interval

LeetCode Problem 1124 Difficulty: 🟡 Medium Topics: Array, Hash Table, Stack, Monotonic Stack, Prefix Sum Solution Edit LeetCode 1124 - Longest Well-Performing Interval Problem Understanding The problem gives us an array hours , where each element represents the number of hours an employee worked on a specific day. A day is classified as tiring if the employee worked strictly more than 8 hours . Otherwise, the day is considered non-tiring...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 321 - Create Maximum Number

LeetCode Problem 321 Difficulty: 🔴 Hard Topics: Array, Two Pointers, Stack, Greedy, Monotonic Stack Solution LeetCode 321 - Create Maximum Number Problem Understanding This problem asks us to build the lexicographically largest possible number of length k using digits taken from two arrays, nums1 and nums2 . Each array represents a sequence of digits from a number. We are allowed to choose some digits from nums1 and some digits from...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 316 - Remove Duplicate Letters

LeetCode Problem 316 Difficulty: 🟡 Medium Topics: String, Stack, Greedy, Monotonic Stack Solution LeetCode 316, Remove Duplicate Letters Problem Understanding This problem asks us to construct a new string from the input string s such that every distinct character appears exactly once, while also ensuring that the final result is the smallest possible in lexicographical order. The input is a string consisting only of lowercase English letters. Characters may appear...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1776 - Car Fleet II

LeetCode Problem 1776 Difficulty: 🔴 Hard Topics: Array, Math, Stack, Heap (Priority Queue), Monotonic Stack Solution Problem Understanding The problem is asking us to compute the time at which each car in a line of cars collides with the car immediately in front of it, or return -1 if it never collides. Each car is represented by its position and speed. The cars are initially sorted by position in increasing...

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LeetCode 385 - Mini Parser

LeetCode Problem 385 Difficulty: 🟡 Medium Topics: String, Stack, Depth-First Search Solution Problem Understanding The problem gives us a string representation of a nested integer structure and asks us to reconstruct the corresponding NestedInteger object hierarchy. A NestedInteger can represent one of two things: A single integer value A list of other NestedInteger objects The input string uses a format similar to JSON arrays. For example: "324" represents a single...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 394 - Decode String

LeetCode Problem 394 Difficulty: 🟡 Medium Topics: String, Stack, Recursion Solution LeetCode 394 - Decode String Problem Understanding The problem gives us a string that contains encoded patterns of the form k[encoded_string] . The integer k tells us how many times the substring inside the brackets should be repeated. Our task is to fully decode the string and return the expanded result. For example, the input: 3[a] means the string...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 962 - Maximum Width Ramp

LeetCode Problem 962 Difficulty: 🟡 Medium Topics: Array, Two Pointers, Stack, Monotonic Stack Solution Problem Understanding The problem asks us to find the maximum width ramp in an array of integers. A ramp is defined as a pair of indices (i, j) such that: i < j nums[i] <= nums[j] The width of the ramp is simply the distance between the two indices: $$\text{width} = j - i$$ Our goal...

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LeetCode 1019 - Next Greater Node In Linked List

LeetCode Problem 1019 Difficulty: 🟡 Medium Topics: Array, Linked List, Stack, Monotonic Stack Solution Problem Understanding The problem asks us to process a singly-linked list and, for each node, determine the value of the next node that has a strictly larger value. Concretely, if the list is [2,1,5] , for the first node 2 , the next larger node is 5 ; for the second node 1 , the next...

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LeetCode 456 - 132 Pattern

LeetCode Problem 456 Difficulty: 🟡 Medium Topics: Array, Binary Search, Stack, Monotonic Stack, Ordered Set Solution Problem Understanding The problem asks us to determine whether an array contains a specific ordering pattern called a "132 pattern". A valid 132 pattern consists of three indices i , j , and k such that: i < j < k nums[i] < nums[k] < nums[j] The name "132" comes from the relative ordering...

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LeetCode 1381 - Design a Stack With Increment Operation

LeetCode Problem 1381 Difficulty: 🟡 Medium Topics: Array, Stack, Design Solution Problem Understanding This problem requires designing a specialized stack data structure, called CustomStack , that not only supports the usual push and pop operations but also an incremental operation that can increase the value of the bottom k elements by a given amount. In other words, this stack behaves like a standard stack for pushing and popping elements, but...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 975 - Odd Even Jump

LeetCode Problem 975 Difficulty: 🔴 Hard Topics: Array, Dynamic Programming, Stack, Sorting, Monotonic Stack, Ordered Set Solution Problem Understanding The problem asks us to determine how many starting indices in an array allow reaching the last element by performing a series of jumps defined as either odd-numbered or even-numbered. The distinction between odd and even jumps is crucial: odd-numbered jumps must move to the smallest value greater than or equal...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1214 - Two Sum BSTs

LeetCode Problem 1214 Difficulty: 🟡 Medium Topics: Two Pointers, Binary Search, Stack, Tree, Depth-First Search, Binary Search Tree, Binary Tree Solution Problem Understanding The problem gives us two binary search trees, root1 and root2 , along with an integer target . We must determine whether there exists one node from the first tree and one node from the second tree such that their values add up exactly to target ....

Practice › LeetCode › LeetCode 03xx ›

LeetCode 331 - Verify Preorder Serialization of a Binary Tree

LeetCode Problem 331 Difficulty: 🟡 Medium Topics: String, Stack, Tree, Binary Tree Solution Problem Understanding The problem gives us a string that represents the preorder serialization of a binary tree. Each value is separated by commas. A normal integer represents a real tree node, while the character '#' represents a null pointer. In preorder traversal, we always visit nodes in this order: Current node Left subtree Right subtree For example,...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 769 - Max Chunks To Make Sorted

LeetCode Problem 769 Difficulty: 🟡 Medium Topics: Array, Stack, Greedy, Sorting, Monotonic Stack Solution Problem Understanding This problem is asking us to determine the maximum number of contiguous chunks into which we can split an array such that sorting each chunk individually and concatenating them results in a fully sorted array. The input array arr is a permutation of integers from 0 to n-1 , meaning it contains each integer...

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LeetCode 1130 - Minimum Cost Tree From Leaf Values

LeetCode Problem 1130 Difficulty: 🟡 Medium Topics: Array, Dynamic Programming, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem asks us to construct a binary tree from an input array arr of positive integers, where each integer represents a leaf node in an in-order traversal. Every non-leaf node has exactly two children, and its value is calculated as the product of the largest leaf values in its left and right...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 114 - Flatten Binary Tree to Linked List

LeetCode Problem 114 Difficulty: 🟡 Medium Topics: Linked List, Stack, Tree, Depth-First Search, Binary Tree Solution Problem Understanding The problem asks us to transform a binary tree into a flattened structure that behaves like a singly linked list. The transformation must happen in-place, meaning we are not supposed to create an entirely new tree or list structure. Instead, we must rearrange the existing pointers inside the tree nodes. The flattened...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 682 - Baseball Game

LeetCode Problem 682 Difficulty: 🟢 Easy Topics: Array, Stack, Simulation Solution Problem Understanding This problem asks us to simulate a baseball scoring system with a set of unusual rules. We begin with an empty score record and process a sequence of operations one by one. Each operation either adds a new score or modifies the history of previously recorded scores. The input is an array of strings called operations ....

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1504 - Count Submatrices With All Ones

LeetCode Problem 1504 Difficulty: 🟡 Medium Topics: Array, Dynamic Programming, Stack, Matrix, Monotonic Stack Solution Problem Understanding The problem gives us an m x n binary matrix where each cell contains either 0 or 1 . We must count how many rectangular submatrices consist entirely of ones. A submatrix is any contiguous rectangular region inside the matrix. For every possible rectangle, we need to determine whether every element inside it...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 364 - Nested List Weight Sum II

LeetCode Problem 364 Difficulty: 🟡 Medium Topics: Stack, Depth-First Search, Breadth-First Search Solution Problem Understanding The problem gives us a nested list structure where each element can either be: A single integer Another nested list containing additional integers or lists The goal is to compute a weighted sum of all integers, but unlike the standard "Nested List Weight Sum" problem, the weighting is inverted. Normally, deeper integers receive larger weights....

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1673 - Find the Most Competitive Subsequence

LeetCode Problem 1673 Difficulty: 🟡 Medium Topics: Array, Stack, Greedy, Monotonic Stack Solution Here is a complete, detailed technical solution guide for LeetCode 1673 - Find the Most Competitive Subsequence following your formatting rules. Problem Understanding The problem asks us to find the most competitive subsequence of a given integer array nums of size k . A subsequence is formed by deleting zero or more elements from the array without...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 654 - Maximum Binary Tree

LeetCode Problem 654 Difficulty: 🟡 Medium Topics: Array, Divide and Conquer, Stack, Tree, Monotonic Stack, Binary Tree Solution Problem Understanding The problem asks us to construct a maximum binary tree from a given integer array nums with unique elements. A maximum binary tree is a binary tree where each node is the maximum element of the subarray it represents. Specifically, the root of the tree is the largest element in...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 853 - Car Fleet

LeetCode Problem 853 Difficulty: 🟡 Medium Topics: Array, Stack, Sorting, Monotonic Stack Solution Problem Understanding The problem describes a set of cars driving toward the same destination, represented by the integer target . Each car starts at a unique position and moves at a constant speed. Cars cannot overtake each other. If a faster car catches up to a slower car ahead of it, the faster car must slow down...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 770 - Basic Calculator IV

LeetCode Problem 770 Difficulty: 🔴 Hard Topics: Hash Table, Math, String, Stack, Recursion Solution Problem Understanding The problem requires parsing and evaluating a mathematical expression that includes integers, variables, addition, subtraction, multiplication, and parentheses, while also applying a substitution map for certain variables. Essentially, we are building a symbolic algebra engine that simplifies expressions according to standard arithmetic rules. The input consists of a string expression that may contain integers,...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 907 - Sum of Subarray Minimums

LeetCode Problem 907 Difficulty: 🟡 Medium Topics: Array, Dynamic Programming, Stack, Monotonic Stack Solution Problem Understanding The problem asks us to compute the sum of the minimum value of every possible contiguous subarray of a given array. For an array arr , every contiguous slice of the array is considered a subarray. For each subarray, we determine its minimum element. The final answer is the sum of all of those...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 224 - Basic Calculator

LeetCode Problem 224 Difficulty: 🔴 Hard Topics: Math, String, Stack, Recursion Solution Problem Understanding The problem gives us a string representing a mathematical expression containing integers, addition, subtraction, parentheses, and spaces. Our task is to evaluate the expression and return the final integer result. The expression is guaranteed to be valid. That guarantee is extremely important because it simplifies parsing. We do not need to worry about malformed parentheses, invalid...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1526 - Minimum Number of Increments on Subarrays to Form a Target Array

LeetCode Problem 1526 Difficulty: 🔴 Hard Topics: Array, Dynamic Programming, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem gives us a target array and asks us to build it starting from an array of all zeros. The only operation allowed is selecting any contiguous subarray and incrementing every element in that subarray by exactly one. The goal is to determine the minimum number of such operations required to transform...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 234 - Palindrome Linked List

LeetCode Problem 234 Difficulty: 🟢 Easy Topics: Linked List, Two Pointers, Stack, Recursion Solution Problem Understanding The problem gives us the head of a singly linked list and asks whether the sequence of values stored in the list forms a palindrome. A palindrome is a sequence that reads the same forward and backward. For example, the array [1,2,2,1] is a palindrome because reversing it produces the same sequence. On the...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1653 - Minimum Deletions to Make String Balanced

LeetCode Problem 1653 Difficulty: 🟡 Medium Topics: String, Dynamic Programming, Stack Solution Problem Understanding The problem asks us to make a string containing only characters 'a' and 'b' balanced by deleting the minimum number of characters. A string is considered balanced if there are no occurrences where a 'b' comes before an 'a' in the string. In other words, all 'a' characters must appear before any 'b' characters, or equivalently,...

Practice › LeetCode › LeetCode 12xx ›

LeetCode 1265 - Print Immutable Linked List in Reverse

LeetCode Problem 1265 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers, Stack, Recursion Solution Problem Understanding This problem gives us a special type of linked list called an immutable linked list. Unlike a normal linked list problem, we are not allowed to access node fields directly. Instead, we can only interact with the list through two provided APIs: printValue() , which prints the current node's value getNext() , which returns...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 439 - Ternary Expression Parser

LeetCode Problem 439 Difficulty: 🟡 Medium Topics: String, Stack, Recursion Solution Problem Understanding The problem gives us a string representing a nested ternary expression and asks us to evaluate it. A ternary expression follows the familiar format: condition ? value_if_true : value_if_false The condition is always either 'T' or 'F' . If the condition is 'T' , the expression evaluates to the value before the ':' . If the condition...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 736 - Parse Lisp Expression

LeetCode Problem 736 Difficulty: 🔴 Hard Topics: Hash Table, String, Stack, Recursion Solution Problem Understanding The problem asks us to evaluate a Lisp-like expression represented as a string. The expression can contain integers, variables, and three special operations: let , add , and mult . An expression evaluates to a single integer value. The challenge is not arithmetic itself, but correctly handling nested scopes and variable shadowing. A let expression...

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LeetCode 1111 - Maximum Nesting Depth of Two Valid Parentheses Strings

LeetCode Problem 1111 Difficulty: 🟡 Medium Topics: String, Stack Solution Problem Understanding The problem asks us to take a valid parentheses string seq and split it into two disjoint subsequences A and B such that each subsequence is itself a valid parentheses string (VPS). The goal is to assign each character in seq to either A or B in a way that minimizes the maximum nesting depth between the two...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1614 - Maximum Nesting Depth of the Parentheses

LeetCode Problem 1614 Difficulty: 🟢 Easy Topics: String, Stack Solution LeetCode 1614, Maximum Nesting Depth of the Parentheses Problem Understanding This problem asks us to compute the maximum nesting depth of parentheses in a valid parentheses string. A valid parentheses string, often abbreviated as VPS, is a string where every opening parenthesis '(' has a matching closing parenthesis ')' , and the parentheses are properly balanced. The string may also...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 272 - Closest Binary Search Tree Value II

LeetCode Problem 272 Difficulty: 🔴 Hard Topics: Two Pointers, Stack, Tree, Depth-First Search, Binary Search Tree, Heap (Priority Queue), Binary Tree Solution Problem Understanding This problem asks us to find the k values in a Binary Search Tree, or BST, whose values are numerically closest to a given floating point target . A BST has a very important property: Every value in the left subtree is smaller than the current...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 895 - Maximum Frequency Stack

LeetCode Problem 895 Difficulty: 🔴 Hard Topics: Hash Table, Stack, Design, Ordered Set Solution Problem Understanding The problem is asking us to design a custom stack-like data structure that supports two operations: pushing values onto the stack and popping the most frequent element. A key difference from a traditional stack is that the pop operation does not necessarily remove the element at the top; instead, it removes the element with...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 144 - Binary Tree Preorder Traversal

LeetCode Problem 144 Difficulty: 🟢 Easy Topics: Stack, Tree, Depth-First Search, Binary Tree Solution LeetCode 144, Binary Tree Preorder Traversal Problem Understanding This problem asks us to return the preorder traversal of a binary tree. A binary tree consists of nodes where each node may have a left child and a right child. The input root represents the root node of that tree. In a preorder traversal, nodes are visited...

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LeetCode 897 - Increasing Order Search Tree

LeetCode Problem 897 Difficulty: 🟢 Easy Topics: Stack, Tree, Depth-First Search, Binary Search Tree, Binary Tree Solution Problem Understanding This problem gives us the root of a Binary Search Tree, abbreviated as BST, and asks us to rearrange the tree into a very specific form. A Binary Search Tree has the property that for every node: All values in the left subtree are smaller All values in the right subtree...

Practice › LeetCode › LeetCode 09xx ›

LeetCode 946 - Validate Stack Sequences

LeetCode Problem 946 Difficulty: 🟡 Medium Topics: Array, Stack, Simulation Solution Problem Understanding This problem asks whether two sequences of integers, pushed and popped , could represent valid operations on a stack. The pushed array represents the order in which elements are pushed onto an initially empty stack, while the popped array represents the order in which elements are popped from the stack. The task is to determine if the...

Practice › LeetCode › LeetCode 15xx ›

LeetCode 1598 - Crawler Log Folder

LeetCode Problem 1598 Difficulty: 🟢 Easy Topics: Array, String, Stack Solution Problem Understanding In this problem, we are simulating navigation inside a file system. The system starts at the main folder, and we are given a sequence of folder navigation operations. Each operation changes the current directory in one of three ways. The operation "../" means moving up to the parent directory. However, if we are already at the main...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 150 - Evaluate Reverse Polish Notation

LeetCode Problem 150 Difficulty: 🟡 Medium Topics: Array, Math, Stack Solution LeetCode 150, Evaluate Reverse Polish Notation Problem Understanding This problem asks us to evaluate an arithmetic expression written in Reverse Polish Notation, also called postfix notation. Instead of placing operators between operands as in standard infix notation, Reverse Polish Notation places operators after their operands. For example, the infix expression: (2 + 1) * 3 becomes: 2 1 +...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1106 - Parsing A Boolean Expression

LeetCode Problem 1106 Difficulty: 🔴 Hard Topics: String, Stack, Recursion Solution Problem Understanding The problem asks us to evaluate a boolean expression represented as a string . The expression can contain the literals 't' and 'f' for true and false, as well as three types of operators: logical NOT '!' , logical AND '&' , and logical OR '|' . Each operator has a specific syntax: '!(subExpr)' negates a sub-expression,...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 735 - Asteroid Collision

LeetCode Problem 735 Difficulty: 🟡 Medium Topics: Array, Stack, Simulation Solution Problem Understanding The problem gives us a list of integers representing asteroids moving through space in a straight line. Each integer contains two pieces of information: The absolute value represents the asteroid's size. The sign represents the direction: Positive numbers move to the right. Negative numbers move to the left. All asteroids move at the same speed. When two...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 225 - Implement Stack using Queues

LeetCode Problem 225 Difficulty: 🟢 Easy Topics: Stack, Design, Queue Solution Problem Understanding The problem asks us to implement a stack data structure while using only queue operations internally. A stack follows the Last-In-First-Out, or LIFO, principle. This means the most recently inserted element must be removed first. In contrast, a queue follows the First-In-First-Out, or FIFO, principle, where elements leave in the same order they entered. We must design...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1717 - Maximum Score From Removing Substrings

LeetCode Problem 1717 Difficulty: 🟡 Medium Topics: String, Stack, Greedy Solution Problem Understanding The problem gives us a string s and two scoring rules: Removing the substring "ab" earns x points. Removing the substring "ba" earns y points. We may perform these removals as many times as possible, in any order we choose. Every time we remove a substring, the remaining characters close together, which may create new removable substrings....

Practice › LeetCode › LeetCode 07xx ›

LeetCode 768 - Max Chunks To Make Sorted II

LeetCode Problem 768 Difficulty: 🔴 Hard Topics: Array, Stack, Greedy, Sorting, Monotonic Stack Solution Problem Understanding The problem asks us to partition an array into the maximum possible number of contiguous chunks such that, after sorting each chunk independently and concatenating the results, the entire array becomes globally sorted. The key detail is that each chunk must remain contiguous. We cannot rearrange elements across chunk boundaries after splitting. Once the...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 426 - Convert Binary Search Tree to Sorted Doubly Linked List

LeetCode Problem 426 Difficulty: 🟡 Medium Topics: Linked List, Stack, Tree, Depth-First Search, Binary Search Tree, Binary Tree, Doubly-Linked List Solution Problem Understanding The problem asks us to transform a Binary Search Tree, abbreviated as BST, into a sorted circular doubly linked list. The transformation must happen in place, meaning we are not allowed to create entirely new nodes for the linked list. Instead, we reuse the existing tree nodes...

Practice › LeetCode › LeetCode 10xx ›

LeetCode 1006 - Clumsy Factorial

LeetCode Problem 1006 Difficulty: 🟡 Medium Topics: Math, Stack, Simulation Solution Problem Understanding The problem asks us to compute a clumsy factorial for a given positive integer n . A standard factorial multiplies all integers from n down to 1 , but the clumsy factorial modifies the operations: instead of multiplying all numbers, it cycles through '*' , '/' , '+' , and '-' in that order, applying the operations...

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LeetCode 388 - Longest Absolute File Path

LeetCode Problem 388 Difficulty: 🟡 Medium Topics: String, Stack, Depth-First Search Solution Problem Understanding The problem gives us a string representation of a file system hierarchy. Every line represents either a directory or a file. The hierarchy is encoded using newline characters ( \n ) to separate entries and tab characters ( \t ) to indicate nesting depth. A line with no leading tabs exists at depth 0 , meaning...

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LeetCode 1096 - Brace Expansion II

LeetCode Problem 1096 Difficulty: 🔴 Hard Topics: Hash Table, String, Backtracking, Stack, Breadth-First Search, Sorting Solution Problem Understanding The problem asks us to interpret a string expression representing a set of words generated according to a specific grammar and return all distinct words sorted in lexicographical order. The expression can contain lowercase letters, curly braces {} , and commas , . The rules of the grammar are: A single lowercase...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1475 - Final Prices With a Special Discount in a Shop

LeetCode Problem 1475 Difficulty: 🟢 Easy Topics: Array, Stack, Monotonic Stack Solution Problem Understanding The problem presents a scenario where you are buying items in a shop, each with a given price stored in an array prices . For each item at index i , there may exist a discount based on the next item j (where j > i ) such that prices[j] <= prices[i] . If such a...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 402 - Remove K Digits

LeetCode Problem 402 Difficulty: 🟡 Medium Topics: String, Stack, Greedy, Monotonic Stack Solution Problem Understanding The problem gives us a numeric string num and an integer k . We must remove exactly k digits from the number so that the resulting integer is as small as possible. The key detail is that we are not allowed to reorder digits. We may only delete digits while preserving the relative order of...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1063: Number of Valid Subarrays

Problem Restatement We are given an integer array nums . A valid subarray is one where nums[left] <= nums[i] for all left <= i <= right . In other words, the minimum of the subarray is at the leftmost position. Return the number of valid subarrays. The official constraints state that 1 <= nums.length <= 50000 . Input and Output Item Meaning Input Array nums Output Number of subarrays where...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1081: Smallest Subsequence of Distinct Characters

Problem Restatement We are given a string s . We need to return the lexicographically smallest subsequence of s that contains all distinct characters of s exactly once. The official constraints state that 1 <= s.length <= 1000 and s consists of lowercase English letters. Note: This problem is identical to LeetCode 316 (Remove Duplicate Letters). Input and Output Item Meaning Input String s Output Lexicographically smallest subsequence with each...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1047: Remove All Adjacent Duplicates In String

Problem Restatement We are given a string s . In one step, we choose two adjacent equal letters and remove them. We repeat this until no adjacent equal letters remain. Return the final string. The result is always unique regardless of the order of removals. The official constraints state that 1 <= s.length <= 10^5 and s consists only of lowercase English letters. Input and Output Item Meaning Input String...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1003: Check If Word Is Valid After Substitutions

Problem Restatement We are given a string s . A string is valid if we can repeatedly remove the substring "abc" from it until the string becomes empty. We need to return true if s is valid, and false otherwise. The official constraints state that 1 <= s.length <= 2 * 10^4 and s consists only of letters a , b , and c . Input and Output Item Meaning...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1006: Clumsy Factorial

Problem Restatement The clumsy factorial of a positive integer n is defined as follows: Start from n and apply operators * , / , + , - in a fixed cyclic order, descending through integers. For example, clumsy(10) = 10 * 9 / 8 + 7 - 6 * 5 / 4 + 3 - 2 * 1 . Division is integer division truncated toward zero. We need to return...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1021: Remove Outermost Parentheses

Problem Restatement A valid parentheses string is either empty, a concatenation of valid strings, or a valid string wrapped in parentheses. A primitive valid parentheses string is one that cannot be split into two non-empty valid strings. We are given a valid parentheses string s . We need to remove the outermost parentheses of every primitive part in s and return the result. The official constraints state that 1 <=...

Practice › LeetCode › LeetCode (archived) › LeetCode 10xx ›

LeetCode 1019: Next Greater Node In Linked List

Problem Restatement We are given the head of a singly linked list. For each node, we want to find the value of the next node that has a strictly greater value. If no such node exists, the answer for that node is 0 . Return an array of these answers. The official constraints state that the number of nodes is in [1, 10^4] and 1 <= Node.val <= 10^9 ....

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LeetCode 1096: Brace Expansion II

Problem Restatement We are given a string expression representing a set of strings under a brace expansion language: A letter c represents the set {c} . {e1, e2, ...} represents the union of the sets. e1 e2 (concatenation) represents all strings formed by concatenating one from each. Return all strings in the resulting set in sorted order. The official constraints state that 1 <= expression.length <= 60 . Input and...

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LeetCode 921: Minimum Add to Make Parentheses Valid

Problem Restatement We are given a string s containing only parentheses: "(" ")" In one move, we may insert one parenthesis at any position. Return the minimum number of insertions needed to make the string valid. A valid parentheses string is one where every opening parenthesis has a matching closing parenthesis in the correct order. The official statement defines the operation as inserting one parenthesis anywhere in the string. Input...

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LeetCode 844: Backspace String Compare

Problem Restatement We are given two strings s and t . Each string contains lowercase letters and the character "#" . The "#" character acts like a backspace in a text editor. It deletes the character immediately before it. If there is no character before it, it does nothing. Return True if both strings become equal after applying all backspaces. Otherwise, return False . Input and Output Item Meaning Input...

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LeetCode 946: Validate Stack Sequences

Problem Restatement We are given two integer arrays: pushed popped Both arrays contain the same distinct values. pushed gives the order in which values are pushed onto an initially empty stack. popped gives a possible order in which values are popped from the stack. Return True if popped could be produced by valid stack operations. Otherwise, return False . The official statement asks whether popped could result from a sequence...

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LeetCode 895: Maximum Frequency Stack

Problem Restatement Design a data structure called FreqStack . It must support two operations: Operation Meaning push(val) Push integer val onto the stack pop() Remove and return the value with highest frequency If multiple values have the same highest frequency, pop() must remove and return the one that was pushed most recently. So this is like a stack, but priority is based on: Higher frequency first. More recent push first...

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LeetCode 772: Basic Calculator III

Problem Restatement We are given a valid arithmetic expression string s . The expression may contain: non-negative integers + - * / ( ) spaces We need to evaluate the expression and return the integer result. The expression follows normal arithmetic rules: Parentheses are evaluated first. Multiplication and division happen before addition and subtraction. Operators with the same precedence are evaluated from left to right. Integer division truncates toward zero....

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LeetCode 770: Basic Calculator IV

Problem Restatement We are given an algebraic expression as a string. The expression may contain: integers variables + - * parentheses We are also given some variable values through: evalvars evalints For example: evalvars = ["e"] evalints = [1] means: e = 1 We need to simplify the expression after substituting known variables. The result should be returned as a list of string terms. The official format requires variable factors...

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LeetCode 716: Max Stack

Problem Restatement We need to design a data structure called MaxStack . It behaves like a normal stack, but also supports operations related to the maximum value. The required operations are: Operation Meaning push(x) Push x onto the stack pop() Remove and return the top element top() Return the top element without removing it peekMax() Return the maximum element without removing it popMax() Remove and return the maximum element If...

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LeetCode 739: Daily Temperatures

Problem Restatement We are given an array temperatures . Each value represents the temperature on one day. For every day i , we need to find how many days we must wait until a future day has a strictly warmer temperature. If no warmer future day exists, the answer for that day is 0 . The official problem asks for an array answer where answer[i] is the number of days...

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LeetCode 735: Asteroid Collision

Problem Restatement We are given an integer array asteroids . Each integer represents one asteroid. The absolute value is the asteroid size. The sign is its direction: Value Direction Positive Moving right Negative Moving left All asteroids move at the same speed. When two asteroids collide: Case Result Smaller asteroid Explodes Same size Both explode Same direction They never collide We need to return the final state after all collisions....

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LeetCode 636: Exclusive Time of Functions

Problem Restatement We are given n functions with IDs from 0 to n - 1 . We also receive a list of logs. Each log has this format: "{function_id}:{start_or_end}:{timestamp}" For example: "0:start:3" means function 0 starts at the beginning of timestamp 3 . "1:end:5" means function 1 ends at the end of timestamp 5 . The CPU is single-threaded, so only one function runs at a time. A function may...

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LeetCode 907: Sum of Subarray Minimums

Problem Restatement We are given an integer array arr . For every contiguous subarray, take the minimum value of that subarray. Return the sum of all those minimum values. Because the answer can be large, return it modulo: 10**9 + 7 For example: arr = [3, 1, 2, 4] All subarray minimums are: [3] -> 3 [3, 1] -> 1 [3, 1, 2] -> 1 [3, 1, 2, 4] ->...

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LeetCode 682: Baseball Game

Problem Restatement We are given a list of operations called ops . These operations describe scores in a baseball game. We process the operations from left to right and maintain a record of valid scores. The operations are: Operation Meaning Integer x Record a new score x "+" Record a score equal to the sum of the previous two valid scores "D" Record a score equal to double the previous...

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LeetCode 856: Score of Parentheses

Problem Restatement We are given a balanced parentheses string s . The score is defined by three rules: Form Score "()" 1 AB score(A) + score(B) (A) 2 * score(A) Here, A and B are balanced parentheses strings. We need to return the score of s . Input and Output Item Meaning Input A balanced parentheses string s Output The integer score of s Constraint 2 <= s.length <= 50...

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LeetCode 591: Tag Validator

Problem Restatement We are given a string called code . We must determine whether it is a valid code snippet according to several XML-like rules. The string may contain: Start tags: <TAG> End tags: </TAG> CDATA sections: <![CDATA[ ... ]]> Rules: Tag names must contain only uppercase English letters. Tag name length must be between 1 and 9 . Tags must be properly nested. The entire code must be wrapped...

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LeetCode 590: N-ary Tree Postorder Traversal

Problem Restatement We are given the root of an N-ary tree. An N-ary tree is a tree where each node may have any number of children. We need to return the postorder traversal of the tree's node values. In postorder traversal, we visit: All children from left to right. The current node after its children. The input may be empty. If the tree has no nodes, return an empty list....

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LeetCode 589: N-ary Tree Preorder Traversal

Problem Restatement We are given the root of an N-ary tree. An N-ary tree is a tree where each node can have any number of children. We need to return the preorder traversal of the tree's node values. In preorder traversal, we visit: The current node. Its children from left to right. The number of nodes is in the range [0, 10^4] , node values are between 0 and 10^4...

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LeetCode 853: Car Fleet

Problem Restatement There are n cars driving toward the same destination. The destination is at position target . For each car: Data Meaning position[i] Starting position of car i speed[i] Speed of car i A car cannot pass another car ahead of it. If a faster car catches up to a slower car before the destination, they become one fleet. The faster car slows down and follows the slower car....

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LeetCode 678: Valid Parenthesis String

Problem Restatement We are given a string s . The string contains only three characters: Character Meaning '(' Opening parenthesis ')' Closing parenthesis '*' Wildcard character The wildcard '*' can be treated as one of three choices: Choice Meaning '(' Use it as an opening parenthesis ')' Use it as a closing parenthesis empty string Remove it We need to return true if there is some way to replace every...

Practice › LeetCode › LeetCode (archived) › LeetCode 05xx ›

LeetCode 503: Next Greater Element II

Problem Restatement We are given a circular integer array nums . For each element, we need to find the next greater element after it. The word circular means that after the last element, we continue again from the first element. If no greater element exists, the answer for that position is -1 . The official problem asks us to return the next greater number for every element in a circular...

Practice › LeetCode › LeetCode (archived) › LeetCode 09xx ›

LeetCode 901: Online Stock Span

Problem Restatement We need to design a class called StockSpanner . The class receives stock prices one day at a time through the method next(price) . For each new price, we return its stock span. The span is the number of consecutive days ending today where each price was less than or equal to today's price. For example: prices = [100, 80, 60, 70, 60, 75, 85] The spans are:...

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LeetCode 726: Number of Atoms

Problem Restatement We are given a valid chemical formula string. The formula contains atom names, numbers, and parentheses. We need to count how many times each atom appears, then return the result as a string sorted by atom name. An atom name starts with one uppercase letter. It may be followed by lowercase letters. Examples: "H" "Mg" "Ca" A number after an atom means the atom appears that many times....

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LeetCode 496: Next Greater Element I

Problem Restatement We are given two arrays: Array Meaning nums1 Query numbers nums2 A larger reference array Every value in both arrays is unique, and every value in nums1 appears in nums2 . For each value x in nums1 , we need to find the first greater number appearing to the right of x inside nums2 . If no greater value exists, return -1 for that number. Input and Output...

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LeetCode 445: Add Two Numbers II

Problem Restatement We are given two non-empty linked lists. Each linked list represents a non-negative integer. Each node stores one digit, and the digits are stored in forward order. That means the most significant digit comes first. For example: 7 -> 2 -> 4 -> 3 represents: 7243 We need to add the two numbers and return the sum as a linked list, also in forward order. The numbers have...

Practice › LeetCode › LeetCode (archived) › LeetCode 03xx ›

LeetCode 394: Decode String

Problem Restatement We are given an encoded string s . The encoding rule is: k[encoded_string] This means encoded_string should be repeated exactly k times. For example: "3[a]" -> "aaa" "2[bc]" -> "bcbc" The encoding may be nested: "3[a2[c]]" -> "accaccacc" The input is guaranteed to be valid. Brackets are well-formed, repeat counts are positive integers, and digits only appear as repeat counts. The decoded output length will not exceed 10^5...

Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 225: Implement Stack using Queues

Problem Restatement We need to implement a stack using only queues. A stack follows LIFO order: last in, first out A queue follows FIFO order: first in, first out We need to implement the class MyStack with these operations: Method Meaning push(x) Push x onto the stack pop() Remove and return the top element top() Return the top element without removing it empty() Return whether the stack is empty The...

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LeetCode 224: Basic Calculator

Problem Restatement We are given a string s representing a valid arithmetic expression. The expression may contain: Token Meaning Digits Non-negative integers + Addition - Subtraction or unary negative ( Start of a sub-expression ) End of a sub-expression Space Ignored We need to evaluate the expression and return the integer result. We cannot use built-in expression evaluators such as eval . The official statement asks us to implement a...

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LeetCode 316: Remove Duplicate Letters

Problem Restatement We are given a string s containing lowercase English letters. We need to remove duplicate letters so that every distinct letter appears exactly once. Among all valid results, return the lexicographically smallest one. The official constraints are: Constraint Value s.length 1 <= s.length <= 10^4 Characters Lowercase English letters This problem is also the same as LeetCode 1081, Smallest Subsequence of Distinct Characters. ( github.com ) Input and...

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LeetCode 150: Evaluate Reverse Polish Notation

Problem Restatement We are given an array of strings representing an arithmetic expression in Reverse Polish Notation (RPN). Evaluate the expression and return the integer result. Valid operators are: Operator Meaning + Addition - Subtraction * Multiplication / Division Each operand may be an integer or another expression. Division truncates toward zero. LeetCode guarantees that: the input is always valid there is no division by zero the final answer fits...

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LeetCode 173: Binary Search Tree Iterator

Problem Restatement We need to implement a class: BSTIterator It represents an iterator over the inorder traversal of a binary search tree. For a BST, inorder traversal visits values in ascending order. The class must support: Method Meaning BSTIterator(root) Initialize the iterator with the root of the BST next() Move to the next value and return it hasNext() Return whether there is still a next value The pointer starts before...

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LeetCode 94: Binary Tree Inorder Traversal

Problem Restatement We are given the root of a binary tree. We need to return the inorder traversal of the tree's node values. Inorder traversal means: left subtree current node right subtree The official constraints say the number of nodes is between 0 and 100 , and each node value is between -100 and 100 . The follow-up asks for an iterative solution. Input and Output Item Meaning Input Root...

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LeetCode 145: Binary Tree Postorder Traversal

Problem Restatement We are given the root of a binary tree. Return the postorder traversal of its node values. Postorder traversal visits nodes in this order: left subtree -> right subtree -> root If the tree is empty, return an empty list. LeetCode also includes a follow-up asking for an iterative solution. Examples Example 1: 1 \ 2 / 3 Traversal order: visit left of 1: none visit right subtree...

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LeetCode 144: Binary Tree Preorder Traversal

Problem Restatement We are given the root of a binary tree. Return the preorder traversal of its node values. Preorder traversal visits nodes in this order: root -> left subtree -> right subtree The tree may be empty. In that case, return an empty list. LeetCode also includes a follow-up asking for an iterative solution. Examples Example 1: 1 \ 2 / 3 Preorder traversal: visit 1 go right to...

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LeetCode 71: Simplify Path

Problem Restatement We are given a Unix-style absolute file path. We need to simplify it into its canonical form. The rules are: A single dot "." means the current directory. A double dot ".." means move to the parent directory. Multiple consecutive slashes should be treated as a single slash. Any other name should be treated as a normal directory name. The result must: Start with a single slash Have...

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LeetCode 439: Ternary Expression Parser

Problem Restatement We are given a valid string expression representing nested ternary expressions. A ternary expression has the form: condition ? true_value : false_value The condition is always either: T F where T means true and F means false. The expression contains only: Character Meaning T true F false 0 to 9 single digit value ? starts a ternary choice : separates true branch and false branch Each number has...

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LeetCode 388: Longest Absolute File Path

Problem Restatement We are given a string input that represents a file system. Each file or directory appears on its own line. A newline character means a new entry: "\n" A tab character at the start of a line tells us the depth: "\t" A file is identified by having a dot . in its name. We need to return the length of the longest absolute path to any file....

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LeetCode 341: Flatten Nested List Iterator

Problem Restatement We are given a nested list of integers. Each element is either: Type Meaning Integer A single integer List A nested list whose elements may also be integers or lists We need to implement an iterator that returns the integers in flattened left-to-right order. The class must support: Method Meaning NestedIterator(nestedList) Initializes the iterator next() Returns the next integer hasNext() Returns true if there are more integers LeetCode...

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LeetCode 85: Maximal Rectangle

Problem Restatement We are given a binary matrix filled with characters: "0" "1" We need to find the largest rectangle that contains only "1" cells and return its area. The rectangle must be axis-aligned. That means its sides follow the matrix rows and columns. The official problem states: given a rows x cols binary matrix filled with 0 s and 1 s, find the largest rectangle containing only 1 s...

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LeetCode 84: Largest Rectangle in Histogram

Problem Restatement We are given an integer array heights . Each value represents the height of one bar in a histogram. Each bar has width 1 . We need to return the area of the largest rectangle that can be formed inside the histogram. The rectangle must use consecutive bars. Its height is limited by the shortest bar in that chosen range. The official statement gives heights = [2, 1,...

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LeetCode 484: Find Permutation

Problem Restatement We are given a string s made only of two characters: Character Meaning I The next number must be greater D The next number must be smaller We need to return the lexicographically smallest permutation of numbers from 1 to n , where: n = len(s) + 1 The permutation must match the pattern in s . For each index i : Pattern Required relation s[i] == "I"...

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LeetCode 385: Mini Parser

Problem Restatement We are given a string s . The string represents either: A single integer A nested list of integers Examples: "324" "[123,[456,[789]]]" "[-1,[2,-3],4]" We need to parse this string and return a NestedInteger object. The NestedInteger class is already provided by LeetCode. We only need to implement: deserialize(s) Input and Output Item Meaning Input A serialized string s Output A NestedInteger object Integer format May be positive or...

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LeetCode 232: Implement Queue using Stacks

Problem Restatement We need to implement a queue using only standard stack operations. The queue must support: Operation Meaning push(x) Add x to the back of the queue pop() Remove and return the front element peek() Return the front element without removing it empty() Return whether the queue is empty A queue follows FIFO order: First In, First Out A stack follows LIFO order: Last In, First Out The challenge...

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LeetCode 32: Longest Valid Parentheses

Problem Restatement We are given a string s containing only two characters: "(" ")" We need to return the length of the longest valid parentheses substring. A valid parentheses substring must be continuous, balanced, and well-formed. Examples of valid strings: "()" "(())" "()()" "(()())" Examples of invalid strings: "(" ")(" "(()" "())" If there is no valid parentheses substring, return 0 . The constraints allow an empty string, and the...

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LeetCode 230: Kth Smallest Element in a BST

Problem Restatement We are given the root of a binary search tree and an integer k . We need to return the k th smallest value among all node values in the tree. The counting is 1-indexed. So: k = 1 means the smallest value k = 2 means the second smallest value k = 3 means the third smallest value LeetCode states that 1 <= k <= n <=...

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LeetCode 456: 132 Pattern

Problem Restatement We are given an integer array nums . We need to determine whether there exists a subsequence of three elements: nums[i], nums[j], nums[k] such that: i < j < k and: nums[i] < nums[k] < nums[j] This is called a 132 pattern . The order of indices is 1 -> 3 -> 2 . The values must look like: small < middle < large but they appear in...

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LeetCode 20: Valid Parentheses

Problem Restatement We are given a string s containing only these six bracket characters: ( ) { } [ ] We need to determine whether the bracket string is valid. A string is valid when: Every opening bracket is closed by the same type of bracket. Opening brackets are closed in the correct order. Every closing bracket has a matching opening bracket of the same type. For example: "()[]{}" is...

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LeetCode 331: Verify Preorder Serialization of a Binary Tree

Problem Restatement We are given a string preorder . The string represents a preorder traversal serialization of a binary tree. Each token is separated by a comma. A non-null node is written as an integer. A null child is written as # . For example: "9,3,4,#,#,1,#,#,2,#,6,#,#" We need to return true if this string is a valid preorder serialization of some binary tree. We are not allowed to reconstruct the...

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LeetCode 155: Min Stack

Problem Restatement We need to design a stack that supports normal stack operations and also returns the minimum element currently inside the stack. The class must support these methods: Method Meaning MinStack() Initialize the stack push(val) Push val onto the stack pop() Remove the top element top() Return the top element getMin() Return the minimum element in the stack Each operation must run in O(1) time. LeetCode states that pop...

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LeetCode 255: Verify Preorder Sequence in Binary Search Tree

Problem Restatement We are given an array of unique integers called preorder . We need to decide whether this array could be the preorder traversal of a binary search tree. In preorder traversal, nodes are visited in this order: Visit root. Visit left subtree. Visit right subtree. For a binary search tree: Position Rule Left subtree Values are smaller than the root Right subtree Values are larger than the root...

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LeetCode 402: Remove K Digits

Problem Restatement We are given a string num representing a non-negative integer and an integer k . We must remove exactly k digits from num . After removing those digits, the remaining digits must stay in their original relative order. Return the smallest possible integer as a string. The result should not contain leading zeroes. If all digits are removed, or the remaining number becomes empty after removing leading zeroes,...

Practice › LeetCode › LeetCode (archived) › LeetCode 02xx ›

LeetCode 227: Basic Calculator II

Problem Restatement We are given a string s representing a mathematical expression. The expression contains: Non-negative integers Operators: + , - , * , / Spaces We must evaluate the expression and return the integer result. Division truncates toward zero. LeetCode examples: Input: "3+2*2" Output: 7 Input: " 3/2 " Output: 1 Input: " 3+5 / 2 " Output: 5 The expression is always valid. Input and Output Item Meaning...

Wiki › Algorithms › 02. Data Structures › 1. Core Data Structures and Operations ›

1.3 Stacks

1.3 Stacks, 25 index slug name 1 array-stack Array Stack 2 linked-stack Linked Stack 3 dynamic-stack Dynamic Stack 4 stack-push-pop Push Pop 5 stack-min Min Stack 6 stack-max Max Stack 7 stack-two-in-one Two Stacks 8 stack-k-in-one K Stacks 9 stack-using-queue Stack via Queue 10 stack-recursion Recursion Stack 11 stack-call-frame Call Stack 12 stack-evaluation Expression Eval 13 stack-parentheses Parentheses Check 14 stack-monotonic Monotonic Stack 15 stack-span Stock Span 16 stack-histogram Histogram...

#stack

LeetCode 3430 - Maximum and Minimum Sums of at Most Size K Subarrays

LeetCode 3878 - Count Good Subarrays

LeetCode 3834 - Merge Adjacent Equal Elements

LeetCode 3749 - Evaluate Valid Expressions

LeetCode 3746 - Minimum String Length After Balanced Removals

LeetCode 3816 - Lexicographically Smallest String After Deleting Duplicate Characters

LeetCode 3703 - Remove K-Balanced Substrings

LeetCode 3555 - Smallest Subarray to Sort in Every Sliding Window

LeetCode 3412 - Find Mirror Score of a String

LeetCode 3676 - Count Bowl Subarrays

LeetCode 3638 - Maximum Balanced Shipments

LeetCode 3561 - Resulting String After Adjacent Removals

LeetCode 3542 - Minimum Operations to Convert All Elements to Zero

LeetCode 3523 - Make Array Non-decreasing

LeetCode 3420 - Count Non-Decreasing Subarrays After K Operations

LeetCode 1963 - Minimum Number of Swaps to Make the String Balanced

LeetCode 2434 - Using a Robot to Print the Lexicographically Smallest String

LeetCode 2865 - Beautiful Towers I

LeetCode 1910 - Remove All Occurrences of a Substring

LeetCode 2216 - Minimum Deletions to Make Array Beautiful

LeetCode 1950 - Maximum of Minimum Values in All Subarrays

LeetCode 2816 - Double a Number Represented as a Linked List

LeetCode 2130 - Maximum Twin Sum of a Linked List

LeetCode 2751 - Robot Collisions

LeetCode 3205 - Maximum Array Hopping Score I

LeetCode 3359 - Find Sorted Submatrices With Maximum Element at Most K

LeetCode 2297 - Jump Game VIII

LeetCode 2524 - Maximum Frequency Score of a Subarray

LeetCode 3170 - Lexicographically Minimum String After Removing Stars

LeetCode 2000 - Reverse Prefix of Word

LeetCode 2355 - Maximum Number of Books You Can Take

LeetCode 2019 - The Score of Students Solving Math Expression

LeetCode 2281 - Sum of Total Strength of Wizards

LeetCode 2211 - Count Collisions on a Road

LeetCode 2282 - Number of People That Can Be Seen in a Grid

LeetCode 2645 - Minimum Additions to Make Valid String

LeetCode 2940 - Find Building Where Alice and Bob Can Meet

LeetCode 2116 - Check if a Parentheses String Can Be Valid

LeetCode 3221 - Maximum Array Hopping Score II

LeetCode 2375 - Construct Smallest Number From DI String

LeetCode 2104 - Sum of Subarray Ranges

LeetCode 2696 - Minimum String Length After Removing Substrings

LeetCode 2487 - Remove Nodes From Linked List

LeetCode 2589 - Minimum Time to Complete All Tasks

LeetCode 2345 - Finding the Number of Visible Mountains

LeetCode 2863 - Maximum Length of Semi-Decreasing Subarrays

LeetCode 2813 - Maximum Elegance of a K-Length Subsequence

LeetCode 1896 - Minimum Cost to Change the Final Value of Expression

LeetCode 2764 - Is Array a Preorder of Some ‌Binary Tree

LeetCode 1966 - Binary Searchable Numbers in an Unsorted Array

LeetCode 3174 - Clear Digits

LeetCode 2945 - Find Maximum Non-decreasing Array Length

LeetCode 2454 - Next Greater Element IV

LeetCode 2818 - Apply Operations to Maximize Score

LeetCode 2334 - Subarray With Elements Greater Than Varying Threshold

LeetCode 2296 - Design a Text Editor

LeetCode 2617 - Minimum Number of Visited Cells in a Grid

LeetCode 1944 - Number of Visible People in a Queue

LeetCode 2030 - Smallest K-Length Subsequence With Occurrences of a Letter

LeetCode 3113 - Find the Number of Subarrays Where Boundary Elements Are Maximum

LeetCode 1996 - The Number of Weak Characters in the Game

LeetCode 2866 - Beautiful Towers II

LeetCode 2832 - Maximal Range That Each Element Is Maximum in It

LeetCode 2390 - Removing Stars From a String

LeetCode 2197 - Replace Non-Coprime Numbers in Array

LeetCode 2736 - Maximum Sum Queries

LeetCode 3229 - Minimum Operations to Make Array Equal to Target

LeetCode 1856 - Maximum Subarray Min-Product

LeetCode 2289 - Steps to Make Array Non-decreasing

LeetCode 94 - Binary Tree Inorder Traversal

LeetCode 232 - Implement Queue using Stacks

LeetCode 84 - Largest Rectangle in Histogram

LeetCode 581 - Shortest Unsorted Continuous Subarray

LeetCode 20 - Valid Parentheses

LeetCode 1021 - Remove Outermost Parentheses

LeetCode 143 - Reorder List

LeetCode 1172 - Dinner Plate Stacks

LeetCode 1249 - Minimum Remove to Make Valid Parentheses

LeetCode 1081 - Smallest Subsequence of Distinct Characters