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Practice › LeetCode › LeetCode (archived) › LeetCode 08xx ›

LeetCode 874: Walking Robot Simulation

Problem Restatement A robot starts at: (0, 0) on an infinite XY-plane. It starts facing north. The robot receives commands: Command Meaning -2 Turn left 90 degrees -1 Turn right 90 degrees 1 to 9 Move forward that many units There are also obstacles on the grid. If the robot tries to move onto an obstacle, it stays in its current position and moves to the next command. Return the...

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LeetCode 822: Card Flipping Game

Problem Restatement We are given two arrays: fronts backs Each index represents one card. For card i : fronts[i] is the number on the front side, and: backs[i] is the number on the back side. We may flip any number of cards. A flip swaps the front and back number of that card. After flipping, we choose one card. If the number on the back of that chosen card does...

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LeetCode 820: Short Encoding of Words

Problem Restatement We are given an array of words. A valid encoding uses: Item Meaning s A reference string ending with # indices Starting positions inside s For each word, if we start reading from its index in s and stop before the next # , we must recover that word. For example: s = "time#bell#" indices = [0, 2, 5] This encodes: "time" from index 0 "me" from index...

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LeetCode 817: Linked List Components

Problem Restatement We are given the head of a linked list. Every node in the linked list has a unique integer value. We are also given an integer array nums , where every value in nums appears somewhere in the linked list. Two values from nums are connected if they appear next to each other in the linked list. Return the number of connected components formed by values in nums...

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LeetCode 939: Minimum Area Rectangle

Problem Restatement We are given a list of unique points on a 2D plane. Each point is represented as: [x, y] We need to find the minimum area of a rectangle whose four corners are all in points . The rectangle must have sides parallel to the X-axis and Y-axis. So rotated rectangles do not count. If no such rectangle exists, return: 0 The official statement asks for the minimum...

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LeetCode 694: Number of Distinct Islands

Problem Restatement We are given an m x n binary matrix grid . A cell with value 1 is land. A cell with value 0 is water. An island is a group of land cells connected horizontally or vertically. We need to count how many distinct island shapes exist in the grid. Two islands are considered the same if one can be translated to match the other. Translation means moving...

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LeetCode 663: Equal Tree Partition

Problem Restatement We are given the root of a binary tree. We need to decide whether we can remove exactly one edge so that the tree is split into two smaller trees with equal sums. Each tree sum is the sum of all node values inside that tree. Return True if such an edge exists. Otherwise, return False . Input and Output Item Meaning Input The root of a binary...

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LeetCode 929: Unique Email Addresses

Problem Restatement We are given a list of email addresses. Each email has two parts: local-name@domain-name The problem defines two special rules that apply only to the local name. Rule 1: Ignore Dots In the local name: "." characters are ignored. For example: "alice.z" == "alicez" Rule 2: Ignore Everything After '+' If a plus sign appears in the local name: "+" then everything after it is ignored. For example:...

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LeetCode 804: Unique Morse Code Words

Problem Restatement We are given an array of lowercase English words. Each letter can be converted into Morse code. A word is transformed by replacing every letter with its Morse code and concatenating the results into one string. For example, the word "cab" becomes: "-.-." + ".-" + "-..." = "-.-..--..." We need to return how many different transformed strings appear among all words. The task is to count unique...

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LeetCode 575: Distribute Candies

Problem Restatement We are given an integer array candyType . Each integer represents one candy type. Alice has n candies total, where n = len(candyType) . She is allowed to eat exactly n / 2 candies. Return the maximum number of different candy types Alice can eat. The array length is even. The constraints include n == candyType.length , 1 <= n <= 10^4 , n is even, and -10^5...

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LeetCode 548: Split Array with Equal Sum

Problem Restatement We are given an integer array nums . We need to determine whether there exist indices: i, j, k such that: 0 < i i + 1 < j j + 1 < k k < n - 1 and the following four subarrays all have the same sum: nums[0 : i] nums[i + 1 : j] nums[j + 1 : k] nums[k + 1 : n] The...

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LeetCode 653: Two Sum IV - Input is a BST

Problem Restatement We are given the root of a binary search tree and an integer k . We need to return True if there are two different nodes in the tree whose values add up to k . Otherwise, return False . The two values must come from two different nodes. We cannot use the same node twice. Input and Output Item Meaning Input The root of a binary search...

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LeetCode 752: Open the Lock

Problem Restatement We have a lock with four wheels. Each wheel contains digits from 0 to 9 . The lock starts at: "0000" In one move, we can rotate exactly one wheel by one step. The digits wrap around: 9 -> 0 0 -> 9 Some lock states are deadends. If the lock reaches one of those states, it gets stuck and cannot move anymore. Given a list of deadends...

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LeetCode 472: Concatenated Words

Problem Restatement We are given an array of unique strings: words We need to return all words in the array that can be formed by concatenating at least two shorter words from the same array. The shorter words do not need to be distinct. The same word may be used more than once. The official statement defines a concatenated word as a word comprised entirely of at least two shorter...

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LeetCode 219: Contains Duplicate II

Problem Restatement We are given an integer array nums and an integer k . We need to return True if there are two different indices i and j such that: nums[i] == nums[j] and: abs(i - j) <= k Otherwise, return False . The official statement asks whether two distinct indices have equal values and index distance at most k . Constraints include 1 <= nums.length <= 10^5 , -10^9...

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LeetCode 217: Contains Duplicate

Problem Restatement We are given an integer array nums . We need to determine whether any value appears at least twice. Return: Return value Meaning True Some value appears more than once False Every value is distinct The official statement asks us to return true if any value appears at least twice in the array, and false if every element is distinct. Input and Output Item Meaning Input Integer array...

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LeetCode 140: Word Break II

Problem Restatement We are given: Name Meaning s A string without spaces wordDict A list of valid dictionary words We need to insert spaces into s so that every piece is a valid dictionary word. Return all possible valid sentences. The same dictionary word may be reused multiple times. The answer may be returned in any order. LeetCode gives constraints including 1 <= s.length <= 20 , 1 <= wordDict.length...

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LeetCode 139: Word Break

Problem Restatement We are given: Name Meaning s A string we want to split wordDict A list of valid dictionary words Return True if s can be segmented into a sequence of one or more dictionary words. The same dictionary word may be used multiple times. If no complete segmentation exists, return False . LeetCode states the task as checking whether s can be segmented into a space-separated sequence of...

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LeetCode 433: Minimum Genetic Mutation

Problem Restatement We are given two gene strings: Name Meaning startGene The starting gene endGene The target gene Each gene string has length 8 and contains only these characters: "A", "C", "G", "T" One mutation means changing exactly one character. For example: AACCGGTT -> AACCGGTA This is one mutation because only the last character changed. We are also given a gene bank. A gene is valid only if it appears...

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LeetCode 379: Design Phone Directory

Problem Restatement We need to design a phone directory. The directory starts with maxNumbers available numbers: 0, 1, 2, ..., maxNumbers - 1 We need to support three operations: Operation Meaning get() Return an available number and mark it as used check(number) Return whether number is available release(number) Make a used number available again If no number is available, get() must return -1 . The problem statement defines these operations...

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LeetCode 128: Longest Consecutive Sequence

Problem Restatement We are given an unsorted integer array nums . We need to return the length of the longest consecutive sequence of numbers. The numbers do not need to be adjacent in the array. They only need to exist somewhere in the array. The problem also requires an O(n) time algorithm. Sorting would take O(n log n) , so sorting does not satisfy the required bound. Examples Example 1:...

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LeetCode 353: Design Snake Game

Problem Restatement We need to implement a simplified Snake game. The screen has size: height x width The snake starts at the top-left cell: (0, 0) Its initial length is 1 . We are given a list of food positions. Food appears one at a time. The second food appears only after the snake eats the first food. Each call to move(direction) moves the snake one cell in one of...

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LeetCode 127: Word Ladder

Problem Restatement We are given: Name Meaning beginWord Starting word endWord Target word wordList Dictionary of allowed words We may transform one word into another by changing exactly one character. Every transformed word must exist in wordList . We need to return the number of words in the shortest transformation sequence from beginWord to endWord . If no valid sequence exists, return 0 . Examples Example 1: beginWord = "hit"...

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LeetCode 403: Frog Jump

Problem Restatement A frog wants to cross a river by jumping on stones. The stones are given as a sorted array: stones Each value in stones is a position on the river. The frog starts on the first stone, which is always position 0 . The first jump must be exactly 1 unit. After that, if the frog’s previous jump was k units, then the next jump must be one...

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LeetCode 202: Happy Number

Problem Restatement We are given a positive integer n . We repeatedly replace n with the sum of the squares of its digits. If this process eventually reaches 1 , then n is a happy number. If the process enters a cycle that never reaches 1 , then n is not a happy number. Return True if n is happy. Otherwise, return False . The official problem defines the process...

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LeetCode 126: Word Ladder II

Problem Restatement We are given three things: Name Meaning beginWord The word where the transformation starts endWord The word we want to reach wordList The dictionary of valid transformed words We need to return all shortest transformation sequences from beginWord to endWord . A valid transformation sequence looks like this: beginWord -> s1 -> s2 -> ... -> endWord Each step must obey two rules: Adjacent words must differ by...

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LeetCode 3: Longest Substring Without Repeating Characters

Problem Restatement We are given a string s . We need to find the length of the longest substring that contains no duplicate characters. A substring means a contiguous part of the string. For example, in: s = "pwwkew" "wke" is a substring because its characters appear next to each other. "pwke" is not a substring because it skips one character. It is a subsequence, not a substring. The problem...

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LeetCode 874: Walking Robot Simulation

LeetCode 822: Card Flipping Game

LeetCode 820: Short Encoding of Words

LeetCode 817: Linked List Components

LeetCode 939: Minimum Area Rectangle

LeetCode 694: Number of Distinct Islands

LeetCode 663: Equal Tree Partition

LeetCode 929: Unique Email Addresses

LeetCode 804: Unique Morse Code Words

LeetCode 575: Distribute Candies

LeetCode 548: Split Array with Equal Sum

LeetCode 653: Two Sum IV - Input is a BST

LeetCode 752: Open the Lock

LeetCode 472: Concatenated Words

LeetCode 219: Contains Duplicate II

LeetCode 217: Contains Duplicate

LeetCode 140: Word Break II

LeetCode 139: Word Break

LeetCode 433: Minimum Genetic Mutation

LeetCode 379: Design Phone Directory

LeetCode 128: Longest Consecutive Sequence

LeetCode 353: Design Snake Game

LeetCode 127: Word Ladder

LeetCode 403: Frog Jump

LeetCode 202: Happy Number

LeetCode 126: Word Ladder II

LeetCode 3: Longest Substring Without Repeating Characters