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Practice › LeetCode › LeetCode 35xx ›

LeetCode 3510 - Minimum Pair Removal to Sort Array II

LeetCode Problem 3510 Difficulty: 🔴 Hard Topics: Array, Hash Table, Linked List, Heap (Priority Queue), Simulation, Doubly-Linked List, Ordered Set Solution LeetCode 3510: Minimum Pair Removal to Sort Array II Problem Understanding We are given an array nums . The operation is highly constrained: Among all adjacent pairs, find the pair with the minimum sum. If multiple adjacent pairs have the same minimum sum, choose the leftmost one. Replace those...

Practice › LeetCode › LeetCode 35xx ›

LeetCode 3507 - Minimum Pair Removal to Sort Array I

LeetCode Problem 3507 Difficulty: 🟢 Easy Topics: Array, Hash Table, Linked List, Heap (Priority Queue), Simulation, Doubly-Linked List, Ordered Set Solution LeetCode 3507 - Minimum Pair Removal to Sort Array I Problem Understanding We are given an array nums . At any step, we are not free to choose any adjacent pair. Instead, the operation is completely determined by the current state of the array: Find the adjacent pair whose...

Practice › LeetCode › LeetCode 32xx ›

LeetCode 3294 - Convert Doubly Linked List to Array II

LeetCode Problem 3294 Difficulty: 🟡 Medium Topics: Array, Linked List, Doubly-Linked List Solution Problem Understanding This problem gives us a pointer to an arbitrary node inside a doubly linked list. Unlike many linked list problems, we are not guaranteed to receive the head of the list. Instead, we may receive any node somewhere in the middle or even the tail. A doubly linked list node contains three fields: val ,...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3063 - Linked List Frequency

LeetCode Problem 3063 Difficulty: 🟢 Easy Topics: Hash Table, Linked List, Counting Solution Problem Understanding This problem asks us to analyze a singly-linked list of integers and produce a new linked list containing the frequencies of each distinct element in the input list. Specifically, given the head of a linked list containing k distinct elements, we are to return another linked list of length k where each node represents the...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2095 - Delete the Middle Node of a Linked List

LeetCode Problem 2095 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers Solution Problem Understanding The problem gives us the head of a singly linked list and asks us to delete the middle node. The definition of the middle node is based on 0-based indexing. If the list has n nodes, then the middle node is the node at index ⌊n / 2⌋ . For example: If the list length is...

Practice › LeetCode › LeetCode 22xx ›

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...

Practice › LeetCode › LeetCode 32xx ›

LeetCode 3217 - Delete Nodes From Linked List Present in Array

LeetCode Problem 3217 Difficulty: 🟡 Medium Topics: Array, Hash Table, Linked List Solution Problem Understanding This problem asks us to modify a singly linked list by removing all nodes whose values appear in a given array nums . The input consists of two elements: an array of integers nums and the head of a linked list. Each node in the linked list contains an integer value and a pointer to...

Practice › LeetCode › LeetCode 28xx ›

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...

Practice › LeetCode › LeetCode 21xx ›

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 20xx ›

LeetCode 2046 - Sort Linked List Already Sorted Using Absolute Values

LeetCode Problem 2046 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers, Sorting Solution Problem Understanding The problem provides a singly linked list where the nodes are sorted in non-decreasing order by their absolute values , rather than their actual values. The task is to rearrange this list so that the nodes are sorted in non-decreasing order according to their actual values . For example, consider the input [0, 2, -5,...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2058 - Find the Minimum and Maximum Number of Nodes Between Critical Points

LeetCode Problem 2058 Difficulty: 🟡 Medium Topics: Linked List Solution Problem Understanding The problem gives us a singly linked list and asks us to identify all of its critical points. A critical point is a node that is either a local maximum or a local minimum. A node is considered a local maximum if its value is strictly greater than both its previous node and its next node. Similarly, a...

Practice › LeetCode › LeetCode 21xx ›

LeetCode 2181 - Merge Nodes in Between Zeros

LeetCode Problem 2181 Difficulty: 🟡 Medium Topics: Linked List, Simulation Solution Problem Understanding The problem gives us a singly linked list where the values are grouped between 0 nodes. The list always starts with 0 and ends with 0 , and there are no two consecutive zeros. Every sequence of non-zero nodes between two zeros represents one group. Our task is to replace each group with a single node whose...

Practice › LeetCode › LeetCode 30xx ›

LeetCode 3062 - Winner of the Linked List Game

LeetCode Problem 3062 Difficulty: 🟢 Easy Topics: Linked List Solution Problem Understanding The problem gives us the head of a singly linked list whose length is always even. The nodes are grouped into pairs based on their indices: Nodes at indices (0, 1) form the first pair Nodes at indices (2, 3) form the second pair Nodes at indices (4, 5) form the third pair And so on Within each...

Practice › LeetCode › LeetCode 32xx ›

LeetCode 3263 - Convert Doubly Linked List to Array I

LeetCode Problem 3263 Difficulty: 🟢 Easy Topics: Array, Linked List, Doubly-Linked List Solution Problem Understanding The problem asks us to convert a doubly linked list into a standard integer array while preserving the order of elements. In a doubly linked list, each node contains a value ( val ) and two pointers: next (pointing to the next node) and prev (pointing to the previous node). The input is a reference...

Practice › LeetCode › LeetCode 24xx ›

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...

Practice › LeetCode › LeetCode 26xx ›

LeetCode 2674 - Split a Circular Linked List

LeetCode Problem 2674 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers Solution Problem Understanding The problem asks us to take a circular linked list of positive integers and split it into two separate circular linked lists . The first list should contain the first half of the nodes , rounded up ( ceil(length / 2) ), and the second list should contain the remaining nodes. Importantly, the relative order of...

Practice › LeetCode › LeetCode 23xx ›

LeetCode 2326 - Spiral Matrix IV

LeetCode Problem 2326 Difficulty: 🟡 Medium Topics: Array, Linked List, Matrix, Simulation Solution Problem Understanding The problem gives us a linked list and asks us to place its values into an m x n matrix in clockwise spiral order. We begin filling from the top-left corner of the matrix, which is position (0, 0) . The traversal direction follows the standard spiral pattern: Move left to right across the top...

Practice › LeetCode › LeetCode 20xx ›

LeetCode 2074 - Reverse Nodes in Even Length Groups

LeetCode Problem 2074 Difficulty: 🟡 Medium Topics: Linked List Solution Problem Understanding This problem gives us a singly linked list and asks us to process the nodes in groups whose sizes follow the natural number sequence: Group 1 contains 1 node Group 2 contains 2 nodes Group 3 contains 3 nodes Group 4 contains 4 nodes And so on However, the list may end before a group reaches its intended...

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 28xx ›

LeetCode 2807 - Insert Greatest Common Divisors in Linked List

LeetCode Problem 2807 Difficulty: 🟡 Medium Topics: Linked List, Math, Number Theory Solution LeetCode 2807 - Insert Greatest Common Divisors in Linked List Problem Understanding This problem gives us the head of a singly linked list where every node contains a positive integer. Our task is to modify the list by inserting a new node between every pair of adjacent nodes. The value of each inserted node must be 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 17xx ›

LeetCode 1756 - Design Most Recently Used Queue

LeetCode Problem 1756 Difficulty: 🟡 Medium Topics: Array, Linked List, Divide and Conquer, Design, Simulation, Doubly-Linked List Solution Problem Understanding The problem asks us to design a special queue called an "Most Recently Used Queue", abbreviated as MRUQueue. Initially, the queue contains the integers from 1 to n in increasing order. The key operation is fetch(k) , where k is 1-indexed. This operation does two things: It finds the kth...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 142 - Linked List Cycle II

LeetCode Problem 142 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Two Pointers Solution Problem Understanding This problem asks us to determine whether a singly linked list contains a cycle and, if it does, return the exact node where that cycle begins. A linked list is normally a sequence of nodes where each node points to the next one, eventually ending with null . However, in this problem, the next...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 705 - Design HashSet

LeetCode Problem 705 Difficulty: 🟢 Easy Topics: Array, Hash Table, Linked List, Design, Hash Function Solution Problem Understanding The problem asks us to design our own HashSet implementation without using any built in hash table libraries such as Python's set or Go's built in map type as the primary solution idea. A HashSet is a data structure that stores unique values. It supports three operations: add(key) , insert a value...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 817 - Linked List Components

LeetCode Problem 817 Difficulty: 🟡 Medium Topics: Array, Hash Table, Linked List Solution Problem Understanding The problem gives us a singly linked list where every node contains a unique integer value. We are also given an array nums , and every value in nums is guaranteed to appear somewhere in the linked list. We need to count how many connected components exist among the nodes whose values belong to nums...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 83 - Remove Duplicates from Sorted List

LeetCode Problem 83 Difficulty: 🟢 Easy Topics: Linked List Solution Problem Understanding This problem asks us to remove duplicate values from a sorted singly linked list so that every distinct value appears exactly once. The important detail is that the linked list is already sorted in ascending order, which makes duplicate detection much easier than in an unsorted list. The input is the head of a singly linked list. Each...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 25 - Reverse Nodes in k-Group

LeetCode Problem 25 Difficulty: 🔴 Hard Topics: Linked List, Recursion Solution Problem Understanding This problem asks us to reverse a singly linked list in groups of size k . Instead of reversing the entire list at once, we only reverse consecutive chunks containing exactly k nodes. For example, if the linked list is: 1 -> 2 -> 3 -> 4 -> 5 and k = 2 , we reverse every...

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 02xx ›

LeetCode 237 - Delete Node in a Linked List

LeetCode Problem 237 Difficulty: 🟡 Medium Topics: Linked List Solution Problem Understanding This problem asks us to delete a node from a singly linked list, but with an unusual restriction: we are not given access to the head of the list. Instead, we are only given a reference to the node that should be deleted. In a normal linked list deletion operation, we would traverse the list until we find...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 109 - Convert Sorted List to Binary Search Tree

LeetCode Problem 109 Difficulty: 🟡 Medium Topics: Linked List, Divide and Conquer, Tree, Binary Search Tree, Binary Tree Solution Problem Understanding The problem gives us the head of a singly linked list whose values are already sorted in ascending order. We must convert this linked list into a height balanced Binary Search Tree, usually abbreviated as BST. A Binary Search Tree has the property that: Every node in the left...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 146 - LRU Cache

LeetCode Problem 146 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design an efficient cache that follows the Least Recently Used, or LRU, eviction policy. A cache stores a limited number of key-value pairs. When the cache reaches its maximum capacity and a new item must be inserted, the least recently used item should be removed first. The class...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 432 - All O`one Data Structure

LeetCode Problem 432 Difficulty: 🔴 Hard Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design a special data structure that stores string keys together with their occurrence counts. The structure must support incrementing and decrementing counts, while also being able to instantly retrieve one key with the maximum count and one key with the minimum count. Unlike ordinary frequency counting problems, this...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 430 - Flatten a Multilevel Doubly Linked List

LeetCode Problem 430 Difficulty: 🟡 Medium Topics: Linked List, Depth-First Search, Doubly-Linked List Solution Problem Understanding The problem gives us a special type of doubly linked list where every node contains four fields: val prev next child The next and prev pointers behave exactly like a normal doubly linked list. The extra child pointer introduces another linked list that branches downward from the current node. That child list can itself...

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...

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 00xx ›

LeetCode 23 - Merge k Sorted Lists

LeetCode Problem 23 Difficulty: 🔴 Hard Topics: Linked List, Divide and Conquer, Heap (Priority Queue), Merge Sort Solution Problem Understanding The problem gives us an array of k sorted linked lists. Each linked list is already sorted in ascending order, and our task is to combine all of them into one single sorted linked list. A linked list node contains a value and a pointer to the next node. The...

Practice › LeetCode › LeetCode 18xx ›

LeetCode 1836 - Remove Duplicates From an Unsorted Linked List

LeetCode Problem 1836 Difficulty: 🟡 Medium Topics: Hash Table, Linked List Solution Problem Understanding This problem gives us the head of a singly linked list whose values are not sorted. Our task is to remove every node whose value appears more than once anywhere in the list. The important detail is that we are not removing duplicate occurrences while keeping one copy. Instead, if a value appears multiple times, all...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 708 - Insert into a Sorted Circular Linked List

LeetCode Problem 708 Difficulty: 🟡 Medium Topics: Linked List Solution Let's go step by step and create a complete technical solution guide for LeetCode 708 - Insert into a Sorted Circular Linked List following your formatting requirements. Problem Understanding The problem asks us to insert a value into a sorted circular linked list such that the list remains sorted. A circular linked list is a linked list where the last...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 21 - Merge Two Sorted Lists

LeetCode Problem 21 Difficulty: 🟢 Easy Topics: Linked List, Recursion Solution Problem Understanding This problem asks us to merge two already sorted singly linked lists into one new sorted linked list. The important detail is that we are not creating entirely new nodes for the merged result. Instead, we reuse the existing nodes by reconnecting their next pointers. Each input list is sorted in non-decreasing order. That means values can...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 19 - Remove Nth Node From End of List

LeetCode Problem 19 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers Solution Problem Understanding This problem asks us to remove the nth node counted from the end of a singly linked list and return the modified list. A singly linked list is a sequence of nodes where each node contains a value and a pointer to the next node. Unlike arrays, linked lists do not allow direct indexing, so accessing...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 725 - Split Linked List in Parts

LeetCode Problem 725 Difficulty: 🟡 Medium Topics: Linked List Solution Problem Understanding The problem gives us the head of a singly linked list and an integer k . We must divide the linked list into exactly k consecutive parts while preserving the original order of nodes. The important requirement is that the parts should be as evenly sized as possible. This means: The difference in size between any two parts...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 116 - Populating Next Right Pointers in Each Node

LeetCode Problem 116 Difficulty: 🟡 Medium Topics: Linked List, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution Problem Understanding This problem asks us to populate the next pointers of nodes in a perfect binary tree . A perfect binary tree has all levels completely filled; every internal node has exactly two children, and all leaves are at the same depth. The next pointer of a node should point to the...

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 16xx ›

LeetCode 1634 - Add Two Polynomials Represented as Linked Lists

LeetCode Problem 1634 Difficulty: 🟡 Medium Topics: Linked List, Math, Two Pointers Solution Problem Understanding This problem asks us to add two polynomials represented as singly linked lists. Each node in the linked list represents a single term of a polynomial, with a coefficient and a power. The linked list is sorted in strictly descending order of powers, and terms with zero coefficients do not appear. For example, the polynomial...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 382 - Linked List Random Node

LeetCode Problem 382 Difficulty: 🟡 Medium Topics: Linked List, Math, Reservoir Sampling, Randomized Solution Problem Understanding The problem gives us the head of a singly linked list and asks us to return a random node value such that every node in the list has exactly the same probability of being selected. The class has two operations: Solution(head) initializes the object with the linked list. getRandom() returns one node value chosen...

Practice › LeetCode › LeetCode 03xx ›

LeetCode 369 - Plus One Linked List

LeetCode Problem 369 Difficulty: 🟡 Medium Topics: Linked List, Math Solution Problem Understanding The problem requires us to add one to a non-negative integer that is represented as a singly-linked list. Each node in the list contains a single digit, and the head of the list corresponds to the most significant digit. Essentially, the linked list [1,2,3] represents the integer 123 , and adding one should transform it into [1,2,4]...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 622 - Design Circular Queue

LeetCode Problem 622 Difficulty: 🟡 Medium Topics: Array, Linked List, Design, Queue Solution Problem Understanding The problem asks us to implement a circular queue, a fixed-size queue where the end wraps back to the start to efficiently reuse empty space. In a normal queue implemented with an array, once the array fills, even if elements are dequeued from the front, new elements cannot be enqueued without shifting the array. A...

Practice › LeetCode › LeetCode 01xx ›

LeetCode 138 - Copy List with Random Pointer

LeetCode Problem 138 Difficulty: 🟡 Medium Topics: Hash Table, Linked List Solution Problem Understanding This problem asks us to create a completely independent copy of a linked list where each node contains two pointers: next , which points to the next node in the list random , which can point to any node in the list or null The key requirement is that the copied list must be a true...

Practice › LeetCode › LeetCode 13xx ›

LeetCode 1367 - Linked List in Binary Tree

LeetCode Problem 1367 Difficulty: 🟡 Medium Topics: Linked List, Tree, Depth-First Search, Binary Tree Solution Problem Understanding This problem asks whether a linked list appears as a continuous downward path inside a binary tree. The path does not need to start at the root of the tree, but once it starts, it must move only downward through child pointers, either left or right at each step. The linked list is...

Practice › LeetCode › LeetCode 11xx ›

LeetCode 1171 - Remove Zero Sum Consecutive Nodes from Linked List

LeetCode Problem 1171 Difficulty: 🟡 Medium Topics: Hash Table, Linked List Solution Problem Understanding The problem requires us to repeatedly remove consecutive nodes in a singly-linked list whose values sum to zero. The input is the head of a singly-linked list, where each node has an integer value in the range -1000 to 1000 , and the list length ranges from 1 to 1000 nodes. The output should be the...

Practice › LeetCode › LeetCode 04xx ›

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 00xx ›

LeetCode 24 - Swap Nodes in Pairs

LeetCode Problem 24 Difficulty: 🟡 Medium Topics: Linked List, Recursion Solution Problem Understanding The problem gives us the head of a singly linked list and asks us to swap every pair of adjacent nodes. The important detail is that we are not allowed to modify the values stored inside the nodes. We must physically rearrange the node connections by changing pointers. For example, if the linked list is: 1 ->...

Practice › LeetCode › LeetCode 07xx ›

LeetCode 707 - Design Linked List

LeetCode Problem 707 Difficulty: 🟡 Medium Topics: Linked List, Design Solution LeetCode 707 - Design Linked List Problem Understanding This problem asks us to implement our own linked list data structure from scratch without using any built in linked list library. The linked list must support several common operations such as retrieving a value at a specific index, inserting nodes at different positions, and deleting nodes. The list is 0-indexed,...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1670 - Design Front Middle Back Queue

LeetCode Problem 1670 Difficulty: 🟡 Medium Topics: Array, Linked List, Design, Queue, Doubly-Linked List, Data Stream Solution Problem Understanding The problem is asking us to design a specialized queue that allows insertions and removals not only at the front and back, but also at the middle of the queue. We need to maintain the queue such that we can efficiently access and modify elements at these three positions. The middle...

Practice › LeetCode › LeetCode 17xx ›

LeetCode 1721 - Swapping Nodes in a Linked List

LeetCode Problem 1721 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers Solution Problem Understanding The problem provides a singly linked list and an integer k . The task is to swap the values of the kth node from the start with the kth node from the end of the list. The list is 1-indexed , meaning that the first node is position 1, not 0. The input head represents the...

Practice › LeetCode › LeetCode 08xx ›

LeetCode 876 - Middle of the Linked List

LeetCode Problem 876 Difficulty: 🟢 Easy Topics: Linked List, Two Pointers Solution Problem Understanding The problem gives us the head of a singly linked list and asks us to return the middle node. A singly linked list is a sequence of nodes where each node contains a value and a pointer to the next node. Unlike arrays, linked lists do not support direct indexing, so we cannot instantly jump to...

Practice › LeetCode › LeetCode 10xx ›

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...

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 03xx ›

LeetCode 379 - Design Phone Directory

LeetCode Problem 379 Difficulty: 🟡 Medium Topics: Array, Hash Table, Linked List, Design, Queue Solution Problem Understanding The problem asks us to design a data structure that manages a fixed pool of phone numbers. Initially, every number from 0 to maxNumbers - 1 is available. The directory must support three operations efficiently. The get() operation should assign and return an available number. If no numbers remain, it should return -1...

Practice › LeetCode › LeetCode 16xx ›

LeetCode 1669 - Merge In Between Linked Lists

LeetCode Problem 1669 Difficulty: 🟡 Medium Topics: Linked List Solution Problem Understanding The problem asks us to merge two singly linked lists in a very specific way. We are given list1 and list2 , with sizes n and m respectively, along with two indices a and b . The task is to remove nodes from list1 starting at index a up to index b (inclusive) and insert the entire list2...

Practice › LeetCode › LeetCode 04xx ›

LeetCode 460 - LFU Cache

LeetCode Problem 460 Difficulty: 🔴 Hard Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design a cache that supports two operations: get(key) returns the value associated with a key if it exists, otherwise -1 put(key, value) inserts or updates a key-value pair Unlike a normal cache, eviction is not based only on recency. Instead, this cache uses the Least Frequently Used policy....

Practice › LeetCode › LeetCode 01xx ›

LeetCode 117 - Populating Next Right Pointers in Each Node II

LeetCode Problem 117 Difficulty: 🟡 Medium Topics: Linked List, Tree, Depth-First Search, Breadth-First Search, Binary Tree Solution Problem Understanding The problem asks us to populate the next pointer for every node in a binary tree so that it points to the node immediately to its right on the same level. If no such node exists, the next pointer should remain NULL . Unlike the easier version of this problem, where...

Practice › LeetCode › LeetCode 06xx ›

LeetCode 641 - Design Circular Deque

LeetCode Problem 641 Difficulty: 🟡 Medium Topics: Array, Linked List, Design, Queue Solution Problem Understanding The problem asks us to design a circular double-ended queue, also called a deque. A deque is a data structure that allows insertion and deletion from both the front and the rear. Unlike a normal queue, which only supports enqueue at the back and dequeue at the front, a deque supports operations at both ends...

Practice › LeetCode › LeetCode 00xx ›

LeetCode 86 - Partition List

LeetCode Problem 86 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers Solution It looks like you want the full detailed solution guide, but the specific LeetCode problem number and statement are missing from your latest request template. You included the formatting instructions and structure, but not which problem to solve beyond prior context. Please provide the LeetCode problem number/title (for example, “LeetCode 86 - Partition List”), and I will generate...

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LeetCode 92 - Reverse Linked List II

LeetCode Problem 92 Difficulty: 🟡 Medium Topics: Linked List Solution Problem Understanding This problem asks us to reverse only a specific portion of a singly linked list, rather than reversing the entire list. We are given the head of a linked list and two integer positions, left and right , where left <= right . The goal is to reverse all nodes starting at position left and ending at position...

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LeetCode 706 - Design HashMap

LeetCode Problem 706 Difficulty: 🟢 Easy Topics: Array, Hash Table, Linked List, Design, Hash Function Solution Problem Understanding The problem requires us to implement a HashMap from scratch without using any built-in hash table libraries. In other words, we need to create a data structure that can store key-value pairs, allow efficient insertion, retrieval, and deletion of values based on their keys. The input operations consist of put , get...

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LeetCode 160 - Intersection of Two Linked Lists

LeetCode Problem 160 Difficulty: 🟢 Easy Topics: Hash Table, Linked List, Two Pointers Solution LeetCode 160, Intersection of Two Linked Lists Problem Understanding This problem asks us to determine whether two singly linked lists share a common node, and if they do, return the exact node where the intersection begins. The important detail is that an intersection is based on node reference, not node value. Two nodes containing the same...

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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...

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LeetCode 61 - Rotate List

LeetCode Problem 61 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers Solution Problem Understanding This problem asks us to rotate a singly linked list to the right by k positions. A right rotation means that each node shifts one position toward the end of the list, and the last node wraps around to become the new head. For example, if the linked list is: 1 → 2 → 3 →...

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LeetCode 1797 - Design Authentication Manager

LeetCode Problem 1797 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Design, Doubly-Linked List Solution Problem Understanding The problem asks us to design an authentication system that manages tokens with expiration times. Each token is valid for a fixed timeToLive seconds starting from the moment it is generated or renewed. The system must support three operations: generating a new token, renewing an existing unexpired token, and counting the number of...

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LeetCode 148 - Sort List

LeetCode Problem 148 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers, Divide and Conquer, Sorting, Merge Sort Solution Problem Understanding The problem asks us to sort a singly linked list in ascending order and return the head of the sorted list. The input is the head node of a linked list. Each node contains an integer value and a pointer to the next node. Unlike arrays, linked lists do not...

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LeetCode 1290 - Convert Binary Number in a Linked List to Integer

LeetCode Problem 1290 Difficulty: 🟢 Easy Topics: Linked List, Math Solution Problem Understanding The problem gives us the head of a singly linked list where every node contains either 0 or 1 . These values together represent a binary number. The head of the list stores the most significant bit, meaning the binary digits appear in the same order that we normally write binary numbers from left to right. Our...

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LeetCode 1206 - Design Skiplist

LeetCode Problem 1206 Difficulty: 🔴 Hard Topics: Linked List, Design Solution Problem Understanding This problem asks us to implement a Skiplist from scratch, a probabilistic data structure that allows efficient search, insertion, and deletion operations. A skiplist consists of multiple levels of sorted linked lists, where each higher level acts as an "express lane" to skip over multiple elements in lower levels. The bottom-most level contains all elements in sorted...

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LeetCode 355 - Design Twitter

LeetCode Problem 355 Difficulty: 🟡 Medium Topics: Hash Table, Linked List, Design, Heap (Priority Queue) Solution Problem Understanding This problem asks us to design a simplified version of a social media platform similar to Twitter. The system must support four main operations: Users can post tweets. Users can follow other users. Users can unfollow other users. Users can retrieve a personalized news feed containing the 10 most recent tweets from...

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LeetCode 147 - Insertion Sort List

LeetCode Problem 147 Difficulty: 🟡 Medium Topics: Linked List, Sorting Solution Problem Understanding The problem asks us to sort a singly linked list using the insertion sort algorithm. Unlike array sorting problems where elements can be accessed directly by index, linked lists require sequential traversal, so insertion operations behave differently and require careful pointer manipulation. The input is the head node of a singly linked list. Each node contains an...

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LeetCode 141 - Linked List Cycle

LeetCode Problem 141 Difficulty: 🟢 Easy Topics: Hash Table, Linked List, Two Pointers Solution LeetCode 141, Linked List Cycle Problem Understanding This problem asks us to determine whether a singly linked list contains a cycle. A cycle exists when following the next pointers eventually leads back to a node that has already been visited, instead of reaching None . The input is the head node of a singly linked list....

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LeetCode 328 - Odd Even Linked List

LeetCode Problem 328 Difficulty: 🟡 Medium Topics: Linked List Solution LeetCode 328 - Odd Even Linked List Problem Understanding This problem asks us to reorder a singly linked list so that all nodes located at odd indices appear first, followed by all nodes located at even indices. The important detail is that the grouping is based on the node's position in the list, not the node's value. The first node...

Practice › LeetCode › LeetCode 14xx ›

LeetCode 1474 - Delete N Nodes After M Nodes of a Linked List

LeetCode Problem 1474 Difficulty: 🟢 Easy Topics: Linked List Solution LeetCode 1474, Delete N Nodes After M Nodes of a Linked List Problem Understanding This problem gives us the head of a singly linked list and two integers, m and n . We must traverse the linked list while repeatedly applying a fixed pattern: Keep the next m nodes. Delete the following n nodes. Continue repeating this process until the...

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LeetCode 82 - Remove Duplicates from Sorted List II

LeetCode Problem 82 Difficulty: 🟡 Medium Topics: Linked List, Two Pointers Solution Problem Understanding This problem asks us to process a sorted singly linked list and remove every value that appears more than once. The important distinction is that we are not keeping one copy of duplicated values. Instead, every node containing a duplicated value must be removed entirely. For example, consider the list: 1 -> 2 -> 3 ->...

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LeetCode 2 - Add Two Numbers

LeetCode Problem 2 Difficulty: 🟡 Medium Topics: Linked List, Math, Recursion Solution Add Two Numbers Problem Understanding The problem gives us two non empty singly linked lists. Each linked list represents a non negative integer, but the digits are stored in reverse order. That means the head node contains the least significant digit. For example: [2,4,3] represents the number 342 [5,6,4] represents the number 465 The task is to add...

Practice › LeetCode › LeetCode 02xx ›

LeetCode 206 - Reverse Linked List

LeetCode Problem 206 Difficulty: 🟢 Easy Topics: Linked List, Recursion Solution Problem Understanding The problem asks us to reverse a singly linked list. A singly linked list is a sequence of nodes where each node stores a value and a pointer to the next node in the sequence. The input, head , represents the first node of the linked list. Our task is to rearrange the connections between the nodes...

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LeetCode 203 - Remove Linked List Elements

LeetCode Problem 203 Difficulty: 🟢 Easy Topics: Linked List, Recursion Solution Problem Understanding The problem asks us to remove every node from a singly linked list whose value is equal to a given integer val . After all matching nodes are removed, we must return the head of the modified linked list. A linked list is a linear data structure where each node stores a value and a reference to...

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 ....

Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 725: Split Linked List in Parts

Problem Restatement We are given the head of a singly linked list and an integer k . We need to split the linked list into k consecutive parts. The parts must follow these rules: Rule Meaning Keep original order Nodes must appear in the same order as the input list Sizes as equal as possible No two parts should differ in size by more than 1 Earlier parts are larger...

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

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

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...

Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 708: Insert into a Sorted Circular Linked List

Problem Restatement We are given a node from a sorted circular linked list. The list is sorted in non-decreasing order, but the given node may be any node in the list. It is not necessarily the smallest node. We are also given an integer: insertVal We need to insert a new node with value insertVal into the circular linked list so that the list remains sorted. If the list is...

Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 707: Design Linked List

Problem Restatement We need to design a linked list from scratch. The implementation may use either a singly linked list or a doubly linked list. This guide uses a singly linked list. A singly linked list node stores two things: Field Meaning val The value stored in the node next A reference to the next node The linked list is 0-indexed. We need to implement the MyLinkedList class with these...

Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 706: Design HashMap

Problem Restatement We need to design a class called MyHashMap . The class should support three operations: Operation Meaning put(key, value) Insert or update the value for key get(key) Return the value for key , or -1 if missing remove(key) Remove key and its value if it exists We cannot use built-in hash table libraries. A hash map stores key-value pairs. Each key maps to one value. If put is...

Practice › LeetCode › LeetCode (archived) › LeetCode 07xx ›

LeetCode 705: Design HashSet

Problem Restatement We need to design a class called MyHashSet . The class should support three operations: Operation Meaning add(key) Insert key into the hash set remove(key) Remove key from the hash set if it exists contains(key) Return whether key exists in the hash set We cannot use built-in hash table libraries. The value range is bounded: 0 <= key <= 1_000_000 At most 10,000 calls will be made to...

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

LeetCode 876: Middle of the Linked List

Problem Restatement We are given the head of a singly linked list. We need to return the middle node of the linked list. If the linked list has two middle nodes, we return the second middle node. For example, in a list with 6 nodes, nodes 3 and 4 are both middle candidates, so we return node 4. LeetCode states this rule explicitly. Input and Output Item Meaning Input head...

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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 369: Plus One Linked List

Problem Restatement We are given the head of a non-empty singly linked list. The linked list represents a non-negative integer. Each node stores one digit from 0 to 9 . The digits are stored in normal order, so the most significant digit is at the head. For example: 1 -> 2 -> 3 represents the number: 123 We need to add 1 to this number and return the head of...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 148: Sort List

Problem Restatement We are given the head of a singly linked list. Return the list after sorting it in ascending order. The problem asks for an O(n log n) time solution. The follow-up asks whether we can sort the list with constant extra space. The number of nodes is in the range [0, 5 * 10^4] , and node values are between -10^5 and 10^5 . Examples Example 1: head...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 147: Insertion Sort List

Problem Restatement We are given the head of a singly linked list. Sort the list using insertion sort and return the new head. Insertion sort builds a sorted output list one node at a time. At each step, it removes one node from the input, finds where it belongs in the sorted part, and inserts it there. LeetCode gives examples such as [4,2,1,3] -> [1,2,3,4] and [-1,5,3,4,0] -> [-1,0,3,4,5] ....

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 146: LRU Cache

Problem Restatement Design a data structure that supports: Operation Meaning get(key) Return the value if the key exists, otherwise -1 put(key, value) Insert or update the key-value pair The cache has a fixed capacity. When the cache exceeds capacity, remove the least recently used item. Both operations must run in: O(1) average time. LeetCode defines the least recently used item as the entry that has not been accessed or updated...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 143: Reorder List

Problem Restatement We are given the head of a singly linked list. The list can be written as: L0 -> L1 -> L2 -> ... -> Ln-1 -> Ln We need to reorder it into: L0 -> Ln -> L1 -> Ln-1 -> L2 -> Ln-2 -> ... The reordering must be done in-place. We may change node links, but we may not modify node values. The function returns nothing....

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LeetCode 92: Reverse Linked List II

Problem Restatement We are given the head of a singly linked list and two integers: left right The positions are 1-indexed. We need to reverse the nodes from position left to position right , inclusive, and return the modified list. Only that middle section should be reversed. The nodes before left and after right must stay in their original order. The official statement asks us to reverse the nodes from...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 142: Linked List Cycle II

Problem Restatement We are given the head of a linked list. If the linked list contains a cycle, return the node where the cycle begins. If there is no cycle, return None . LeetCode internally represents the cycle using an index pos , where the tail points back to the node at index pos . This value is not passed into the function. Examples Example 1: 3 -> 2 ->...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 117: Populating Next Right Pointers in Each Node II

Problem Restatement We are given the root of a binary tree. Each node has four fields: val left right next We must populate every next pointer so that it points to the next node on the same level. If there is no node to the right, the next pointer should be None . Unlike LeetCode 116, the tree is not necessarily perfect. Some nodes may have only one child or...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 116: Populating Next Right Pointers in Each Node

Problem Restatement We are given a perfect binary tree. A perfect binary tree has two important properties: Every internal node has exactly two children. All leaves are on the same level. Each node has four fields: val left right next We need to populate each next pointer so that it points to the node immediately to its right on the same level. If there is no node to the right,...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 141: Linked List Cycle

Problem Restatement We are given the head of a linked list. Return True if the linked list contains a cycle. Otherwise, return False . A cycle exists if some node can be reached again by continuously following next pointers. LeetCode internally represents the cycle using an index pos , where the tail node points back to the node at position pos . This value is not passed to the function....

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 114: Flatten Binary Tree to Linked List

Problem Restatement We are given the root of a binary tree. We need to flatten the tree into a linked list in-place. The linked list must follow the same order as preorder traversal: root -> left -> right After flattening: Every node's left child must become None . Every node's right child points to the next node in preorder order. The official problem specifically requires the flattened structure to use...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 138: Copy List with Random Pointer

Problem Restatement We are given the head of a linked list. Each node contains: Field Meaning val Integer value next Pointer to the next node random Pointer to any node in the list or None We need to create a deep copy of the entire list. The copied list must contain entirely new nodes. Every copied node should preserve: The same val The same next structure The same random structure...

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LeetCode 237: Delete Node in a Linked List

Problem Restatement We are given a node in a singly linked list. We are not given the head of the list. We need to delete the given node from the linked list. The important guarantee is that the given node is not the tail node. So node.next always exists. LeetCode defines deletion here as making the linked list behave as if the given node was removed: the value of the...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 109: Convert Sorted List to Binary Search Tree

Problem Restatement We are given the head of a singly linked list whose values are sorted in ascending order. We need to convert this sorted linked list into a height-balanced binary search tree. The official problem defines a height-balanced binary tree as a binary tree where the depth difference between the two subtrees of every node never exceeds 1 . The resulting tree must satisfy the BST property: left subtree...

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LeetCode 86: Partition List

Problem Restatement We are given the head of a linked list and an integer x . We need to reorder the list so that: nodes with value < x come before: nodes with value >= x The relative order inside each group must stay the same. The official statement says to partition the list around x , preserving the original relative order of nodes in both partitions. The constraints allow...

Practice › LeetCode › LeetCode (archived) › LeetCode 00xx ›

LeetCode 61: Rotate List

Problem Restatement We are given the head of a singly linked list and an integer k . We need to rotate the list to the right by k places. A right rotation means: Remove the last node. Move it to the front. Repeat this process k times. The official constraints are 0 <= number of nodes <= 500 , -100 <= Node.val <= 100 , and 0 <= k <=...

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LeetCode 460: LFU Cache

Problem Restatement We need to design a Least Frequently Used cache. The cache supports two operations: get(key) put(key, value) The cache has a fixed capacity . If the cache is full and we need to insert a new key, we remove the least frequently used key. Each key has a use counter. A key's frequency increases when: get(key) is called on it. put(key, value) is called on an existing key....

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

LeetCode 234: Palindrome Linked List

Problem Restatement We are given the head of a singly linked list. We need to return True if the linked list is a palindrome, and False otherwise. A palindrome reads the same forward and backward. For example: [1, 2, 2, 1] is a palindrome. But: [1, 2] is not a palindrome. LeetCode examples include head = [1,2,2,1] , which returns true , and head = [1,2] , which returns false...

Practice › LeetCode › LeetCode (archived) › LeetCode 00xx ›

LeetCode 83: Remove Duplicates from Sorted List

Problem Restatement We are given the head of a sorted linked list. We need to delete duplicate values so that each value appears only once. Unlike LeetCode 82, we do not remove all copies of a duplicated value. We keep one copy. For example: 1 -> 1 -> 2 becomes: 1 -> 2 The official statement says to delete duplicates so that each element appears only once, then return the...

Practice › LeetCode › LeetCode (archived) › LeetCode 01xx ›

LeetCode 160: Intersection of Two Linked Lists

Problem Restatement We are given the heads of two singly linked lists: headA headB We need to return the node where the two linked lists intersect. If the lists do not intersect, return: None Intersection means the two lists share the same node object in memory. It does not mean two nodes merely have the same value. The test cases have no cycles, and the linked lists must keep their...

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

LeetCode 206: Reverse Linked List

Problem Restatement We are given the head of a singly linked list. We need to reverse the list and return the new head. Example: 1 -> 2 -> 3 -> 4 -> 5 becomes: 5 -> 4 -> 3 -> 2 -> 1 The official statement says: given the head of a singly linked list, reverse the list, and return the reversed list. Input and Output Item Meaning Input Head...

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LeetCode 82: Remove Duplicates from Sorted List II

Problem Restatement We are given the head of a sorted linked list. We need to delete every node whose value appears more than once. This is stronger than keeping one copy. If a value appears two or more times, all nodes with that value must be removed. For example: 1 -> 2 -> 3 -> 3 -> 4 -> 4 -> 5 The values 3 and 4 are duplicated, so...

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LeetCode 25: Reverse Nodes in k-Group

Problem Restatement We are given the head of a linked list and an integer k . We need to reverse the nodes of the list k at a time and return the modified list. If the number of remaining nodes at the end is fewer than k , those nodes must stay in the same order. We may not change node values. We may only change node links. For example:...

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LeetCode 24: Swap Nodes in Pairs

Problem Restatement We are given the head of a linked list. We need to swap every two adjacent nodes and return the modified list head. We are not allowed to modify node values. We must change the actual node links. For example: 1 -> 2 -> 3 -> 4 becomes: 2 -> 1 -> 4 -> 3 The problem statement explicitly requires changing nodes themselves instead of only swapping values....

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LeetCode 23: Merge k Sorted Lists

Problem Restatement We are given an array of linked lists: lists Each linked list is sorted in ascending order. We need to merge all linked lists into one sorted linked list and return its head. The constraints say: Constraint Value k == lists.length Number of linked lists 0 <= k <= 10^4 Number of lists 0 <= lists[i].length <= 500 Length of each list Total nodes across all lists At...

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LeetCode 21: Merge Two Sorted Lists

Problem Restatement We are given the heads of two sorted linked lists: list1 list2 Both lists are sorted in non-decreasing order. We need to merge them into one sorted linked list and return the head of the merged list. The merged list should be made by splicing together the nodes of the original two lists. The constraints say the total number of nodes across both lists is in the range...

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LeetCode 432: All O'one Data Structure

Problem Restatement We need to design a data structure that stores string counts. It must support these operations: Operation Meaning inc(key) Increase key count by 1 ; insert it with count 1 if missing dec(key) Decrease key count by 1 ; remove it if count becomes 0 getMaxKey() Return any key with the largest count getMinKey() Return any key with the smallest count If the data structure is empty, both...

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

LeetCode 203: Remove Linked List Elements

Problem Restatement We are given the head of a singly linked list and an integer val . We need to remove every node whose value equals val . Return the new head of the linked list after all removals. The official statement says: given the head of a linked list and an integer val , remove all the nodes of the linked list that have Node.val == val , and...

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LeetCode 382: Linked List Random Node

Problem Restatement We are given the head of a singly linked list. We need to implement a class with one main operation: getRandom() This operation returns the value of a random node from the linked list. Every node must have the same probability of being chosen. The linked list has at least one node, so getRandom() always has a valid value to return. The follow-up asks whether we can solve...

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LeetCode 19: Remove Nth Node From End of List

Problem Restatement We are given the head of a linked list and an integer n . We need to remove the n th node from the end of the list and return the head of the modified list. The follow-up asks whether we can solve it in one pass. The constraints say the list size is sz , where 1 <= sz <= 30 , node values are between 0...

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LeetCode 430: Flatten a Multilevel Doubly Linked List

Problem Restatement We are given the head of a multilevel doubly linked list. Each node has four fields: Field Meaning val Node value prev Previous node on the same level next Next node on the same level child Head of another doubly linked list The child list can also contain nodes with their own child lists. We need to flatten everything into one single-level doubly linked list. When a node...

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

LeetCode 328: Odd Even Linked List

Problem Restatement We are given the head of a singly linked list. We need to reorder the list so that all nodes at odd positions come first, followed by all nodes at even positions. The position is 1-indexed: 1st node -> odd 2nd node -> even 3rd node -> odd 4th node -> even This is about node positions, not node values. The relative order inside each group must stay...

Practice › LeetCode › LeetCode (archived) › LeetCode 04xx ›

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

Problem Restatement We are given the root of a binary search tree. We need to convert the tree into a sorted circular doubly linked list in-place. “In-place” means we should reuse the existing tree nodes. We should not create new list nodes. After conversion: Tree Pointer List Meaning left previous node right next node The returned node should be the smallest node in the list. The list must also be...

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LeetCode 2: Add Two Numbers

Problem Restatement We are given two non-empty linked lists. Each linked list represents a non-negative integer. The digits are stored in reverse order. For example: 2 -> 4 -> 3 actually represents the number: 342 because the least significant digit comes first. We need to add the two numbers together and return the result as a linked list in the same reversed format. Each node contains exactly one digit. Input...

Wiki › Algorithms › 01. Searching and Sorting › 6. Elementary Sorting ›

Strand Sort

Strand Sort Strand sort builds the sorted result by repeatedly extracting increasing subsequences, called strands, from the input and merging them into an output list. It is particularly natural for linked lists, where removing elements is efficient. Problem Given a sequence $A$ of length $n$, reorder it such that $$ A[0] \le A[1] \le \cdots \le A[n-1] $$ Key Idea Take the first element as the start of a strand...

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

1.2 Linked Lists

1.2 Linked lists, 35 index slug name 1 singly-linked-list Singly Linked List 2 doubly-linked-list Doubly Linked List 3 circular-linked-list Circular Linked List 4 sentinel-list Sentinel List 5 intrusive-list Intrusive List 6 skip-pointer-list Skip Pointer List 7 list-insertion Insertion 8 list-deletion Deletion 9 list-reversal Reversal 10 list-cycle-detection Cycle Detection 11 list-merge Merge 12 list-split Split 13 list-concatenation Concatenation 14 list-length Length 15 list-middle Find Middle 16 list-kth-element Kth Element 17 list-copy...

#linked-list

LeetCode 3510 - Minimum Pair Removal to Sort Array II

LeetCode 3507 - Minimum Pair Removal to Sort Array I

LeetCode 3294 - Convert Doubly Linked List to Array II

LeetCode 3063 - Linked List Frequency

LeetCode 2095 - Delete the Middle Node of a Linked List

LeetCode 2296 - Design a Text Editor

LeetCode 3217 - Delete Nodes From Linked List Present in Array

LeetCode 2816 - Double a Number Represented as a Linked List

LeetCode 2130 - Maximum Twin Sum of a Linked List

LeetCode 2046 - Sort Linked List Already Sorted Using Absolute Values

LeetCode 2058 - Find the Minimum and Maximum Number of Nodes Between Critical Points

LeetCode 2181 - Merge Nodes in Between Zeros

LeetCode 3062 - Winner of the Linked List Game

LeetCode 3263 - Convert Doubly Linked List to Array I

LeetCode 2487 - Remove Nodes From Linked List

LeetCode 2674 - Split a Circular Linked List

LeetCode 2326 - Spiral Matrix IV

LeetCode 2074 - Reverse Nodes in Even Length Groups

LeetCode 2289 - Steps to Make Array Non-decreasing

LeetCode 2807 - Insert Greatest Common Divisors in Linked List

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

LeetCode 1756 - Design Most Recently Used Queue

LeetCode 142 - Linked List Cycle II

LeetCode 705 - Design HashSet

LeetCode 817 - Linked List Components

LeetCode 83 - Remove Duplicates from Sorted List

LeetCode 25 - Reverse Nodes in k-Group

LeetCode 716 - Max Stack

LeetCode 237 - Delete Node in a Linked List

LeetCode 109 - Convert Sorted List to Binary Search Tree

LeetCode 146 - LRU Cache

LeetCode 432 - All O`one Data Structure

LeetCode 430 - Flatten a Multilevel Doubly Linked List

LeetCode 143 - Reorder List

LeetCode 234 - Palindrome Linked List

LeetCode 23 - Merge k Sorted Lists

LeetCode 1836 - Remove Duplicates From an Unsorted Linked List

LeetCode 708 - Insert into a Sorted Circular Linked List

LeetCode 21 - Merge Two Sorted Lists

LeetCode 19 - Remove Nth Node From End of List

LeetCode 725 - Split Linked List in Parts

LeetCode 116 - Populating Next Right Pointers in Each Node

LeetCode 1472 - Design Browser History

LeetCode 1634 - Add Two Polynomials Represented as Linked Lists

LeetCode 382 - Linked List Random Node

LeetCode 369 - Plus One Linked List

LeetCode 622 - Design Circular Queue

LeetCode 138 - Copy List with Random Pointer

LeetCode 1367 - Linked List in Binary Tree

LeetCode 1171 - Remove Zero Sum Consecutive Nodes from Linked List

LeetCode 445 - Add Two Numbers II

LeetCode 24 - Swap Nodes in Pairs

LeetCode 707 - Design Linked List

LeetCode 1670 - Design Front Middle Back Queue

LeetCode 1721 - Swapping Nodes in a Linked List

LeetCode 876 - Middle of the Linked List

LeetCode 1019 - Next Greater Node In Linked List

LeetCode 114 - Flatten Binary Tree to Linked List

LeetCode 379 - Design Phone Directory

LeetCode 1669 - Merge In Between Linked Lists

LeetCode 460 - LFU Cache

LeetCode 117 - Populating Next Right Pointers in Each Node II

LeetCode 641 - Design Circular Deque

LeetCode 86 - Partition List

LeetCode 92 - Reverse Linked List II

LeetCode 706 - Design HashMap

LeetCode 160 - Intersection of Two Linked Lists

LeetCode 1265 - Print Immutable Linked List in Reverse

LeetCode 61 - Rotate List

LeetCode 1797 - Design Authentication Manager

LeetCode 148 - Sort List

LeetCode 1290 - Convert Binary Number in a Linked List to Integer

LeetCode 1206 - Design Skiplist

LeetCode 355 - Design Twitter

LeetCode 147 - Insertion Sort List

LeetCode 141 - Linked List Cycle

LeetCode 328 - Odd Even Linked List

LeetCode 1474 - Delete N Nodes After M Nodes of a Linked List

LeetCode 82 - Remove Duplicates from Sorted List II