Given an array of integers, return indices of the two numbers such that they add up to a specific target.

Given an integer array nums, return true if any value appears at least twice in the array, and return false if every element is distinct.

Given an array of strings strs, group the anagrams together. You can return the answer in any order.

Given an integer array nums and an integer k, return the k most frequent elements.

Given an integer array nums, return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[i].

Determine if a 9x9 Sudoku board is valid.

Find the length of the longest consecutive elements sequence in O(n) time.

Given an integer array nums, rotate the array to the right by k steps, where k is non-negative. Do it in-place with O(1) extra space.

Given an integer numRows, return the first numRows of Pascal's triangle. In Pascal's triangle, each number is the sum of the two numbers directly above it.

Given an array nums of size n, return the majority element. The majority element is the element that appears more than ⌊n / 2⌋ times. You may assume that the majority element always exists in the array.

Given an array nums containing n distinct numbers in the range [0, n], return the only number in the range that is missing from the array.

Merge nums2 into nums1 as one sorted array. nums1 has enough space (size m + n) to hold additional elements from nums2.

Given two integer arrays nums1 and nums2, return an array of their intersection. Each element in the result must be unique and you may return the result in any order.

Given an integer array nums of length n where all the integers are in the range [1, n] and each integer appears once or twice, return an array of all the integers that appear twice. You must write an algorithm that runs in O(n) time and uses only constant extra space.

Given an unsorted integer array nums, return the smallest missing positive integer. Must run in O(n) time and O(1) auxiliary space.

Given a list of non-negative integers nums, arrange them such that they form the largest number and return it.

A permutation of an array of integers is an arrangement of its members into a sequence or linear order. The next permutation of an array is the next lexicographically greater permutation. If such arrangement is not possible, it must rearrange it as the lowest possible order (i.e., sorted in ascending order).

Given an m x n board of characters and a list of strings words, return all words on the board.

Given an m x n grid of characters board and a string word, return true if word exists in the grid. The word can be constructed from letters of sequentially adjacent cells, where adjacent cells are horizontally or vertically neighboring. The same letter cell may not be used more than once.

Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target.

Given an array nums of distinct integers, return all the possible permutations. You can return the answer in any order.

Given an integer array nums of unique elements, return all possible subsets (the power set). The solution set must not contain duplicate subsets. Return the solution in any order.

Given an integer array nums that may contain duplicates, return all possible subsets (the power set). The solution set must not contain duplicate subsets. Return the solution in any order.

Given a collection of candidate numbers (candidates) and a target number (target), find all unique combinations in candidates where the candidate numbers sum to target. Each number in candidates may only be used once in the combination.

Given a string s, partition s such that every substring of the partition is a palindrome. Return all possible palindrome partitioning of s.

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent. Return the answer in any order.

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

Find all valid combinations of k numbers that sum up to n such that: Only numbers 1 through 9 are used, each number is used at most once. Return a list of all possible valid combinations.

A valid IP address consists of exactly four integers separated by single dots. Each integer is between 0 and 255 (inclusive) and cannot have leading zeros. Given a string s containing only digits, return all possible valid IP addresses that can be formed by inserting dots into s.

Place n queens on an n x n chessboard such that no two queens attack each other. Return all distinct solutions.

Given two integers n and k, return all possible combinations of k numbers chosen from the range [1, n]. You may return the answer in any order.

Write a program to solve a Sudoku puzzle by filling the empty cells. A sudoku solution must satisfy all of the following rules: Each of the digits 1-9 must occur exactly once in each row, column, and 3x3 sub-box.

Given an array of integers nums which is sorted in ascending order, and an integer target, write a function to search target in nums. If target exists, then return its index. Otherwise, return -1.

Write an efficient algorithm that searches for a value target in an m x n integer matrix matrix.

Given the array nums after the possible rotation and an integer target, return the index of target if it is in nums, or -1 if it is not in nums.

Suppose an array of length n sorted in ascending order is rotated between 1 and n times. Find the minimum element of this array.

Write an efficient algorithm that searches for a value target in an m x n integer matrix matrix. This matrix has the following properties: Integers in each row are sorted in ascending from left to right. Integers in each column are sorted in ascending from top to bottom.

A peak element is an element that is strictly greater than its neighbors. Given a 0-indexed integer array nums, find a peak element, and return its index. If the array contains multiple peaks, return the index to any of the peaks.

You are given a sorted array consisting of only integers where every element appears exactly twice, except for one element which appears exactly once. Return the single element that appears only once.

Given two sorted arrays nums1 and nums2 of size m and n respectively, return the median of the two sorted arrays. The overall run time complexity should be O(log (m+n)).

Given an array of integers nums sorted in non-decreasing order, find the starting and ending position of a given target value. If target is not found, return [-1, -1].

Given a sorted array and a target value, return the index if the target is found. If not, return the index where it would be inserted.

You are a product manager and currently leading a team to develop a new product. Since each version is developed based on the previous version, all the versions after a bad version are also bad. Find the first bad version.

Given a non-empty array of integers nums, every element appears twice except for one. Find that single one. You must implement a solution with linear runtime complexity and use only constant extra space.

Write a function that takes the binary representation of an unsigned integer and returns the number of 1 bits it has (also known as the Hamming weight).

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache. Implement the LRUCache class with get(key) returning the value or -1 if not found, and put(key, value) to insert or update. When capacity is exceeded, evict the least recently used key.

Design and implement a data structure for a Least Frequently Used (LFU) cache. Implement get and put operations with O(1) time complexity.

Given a data stream input of non-negative integers, summarize the numbers seen so far as a list of disjoint intervals.

Given a string s and a dictionary of strings wordDict, return true if s can be segmented into a space-separated sequence of one or more dictionary words.

You are given an integer array coins representing coins of different denominations and an integer amount representing a total amount of money. Return the fewest number of coins that you need to make up that amount. If that amount of money cannot be made up by any combination of the coins, return -1.

Given an integer array nums, return the length of the longest strictly increasing subsequence.

A message containing letters from A-Z can be encoded into numbers using the mapping: 'A' -> "1", 'B' -> "2", ... 'Z' -> "26". To decode an encoded message, all the digits must be grouped then mapped back into letters using the reverse of the mapping above. Given a string s containing only digits, return the number of ways to decode it.

Given two strings text1 and text2, return the length of their longest common subsequence. If there is no common subsequence, return 0.

You are a robber planning to rob houses along a street. Each house has a certain amount of money stashed. All houses at this place are arranged in a circle. That means the first house is the neighbor of the last one. Adjacent houses have security systems connected and will automatically contact police if two adjacent houses were broken into on the same night. Given an integer array nums representing the amount of money of each house, return the maximum amount of money you can rob tonight without alerting the police.

Given a m x n grid filled with non-negative numbers, find a path from top left to bottom right, which minimizes the sum of all numbers along its path. You can only move either down or right at any point in time.

Given two strings word1 and word2, return the minimum number of operations required to convert word1 to word2. Operations: insert, delete, or replace a character.

Given a string s, return the longest palindromic substring in s.

Given strings s1, s2, and s3, determine if s3 is formed by interleaving s1 and s2 while preserving their relative order.

Given an integer array nums, return true if you can partition the array into two subsets such that the sum of the elements in both subsets is equal.

Find the maximum profit with at most two transactions. You may not engage in multiple transactions simultaneously (you must sell the stock before you buy again).

You are given an integer array prices where prices[i] is the price of a given stock on the ith day, and an integer k. Find the maximum profit you can achieve. You may complete at most k transactions. Note: You may not engage in multiple transactions simultaneously (i.e., you must sell the stock before you buy again).

Given a string containing just the characters '(' and ')', return the length of the longest valid (well-formed) parentheses substring.

Given an input string (s) and a pattern (p), implement wildcard pattern matching with support for '?' and '*' where: '?' Matches any single character. '*' Matches any sequence of characters (including the empty sequence).

Given a triangle array, return the minimum path sum from top to bottom. For each step, you may move to an adjacent number of the row below. More formally, if you are on index i on the current row, you may move to either index i or index i + 1 on the next row.

Given an m x n integers matrix, return the length of the longest increasing path in matrix. From each cell, you can either move in four directions: left, right, up, or down. You may not move diagonally or move outside the boundary.

The demons had captured the princess and imprisoned her in the bottom-right corner of a dungeon. The dungeon consists of m x n rooms. Our valiant knight was initially positioned in the top-left room and must fight through the dungeon to rescue the princess. The knight has an initial health point. Some rooms have demons that decrease health, others have magic orbs that increase health. Determine the minimum initial health needed so the knight can rescue the princess.

Given an input string s and a pattern p, implement regular expression matching with support for '.' and '*' where: '.' Matches any single character. '*' Matches zero or more of the preceding element. The matching should cover the entire input string (not partial).

Given a rows x cols binary matrix filled with 0's and 1's, find the largest rectangle containing only 1's and return its area.

Given a string s and a dictionary of strings wordDict, add spaces in s to construct a sentence where each word is a valid dictionary word. Return all such possible sentences in any order.

You want to schedule a list of jobs in d days. Jobs are dependent so you have to finish all jobs from index 0 to i before working on job i+1. Each day you must work on at least one job. The difficulty of a day is the maximum difficulty of a job done that day. Return the minimum sum of difficulties of each day.

Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands.

Given a reference of a node in a connected undirected graph. Return a deep copy (clone) of the graph.

There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take course bi first if you want to take course ai. Return true if you can finish all courses. Otherwise, return false.

There are n cities connected by some number of flights. You are given an array flights where flights[i] = [fromi, toi, pricei] indicates that there is a flight from city fromi to city toi with cost pricei. You are also given three integers src, dst, and k, return the cheapest price from src to dst with at most k stops. If there is no such route, return -1.

You have a graph of n nodes. You are given an integer n and an array edges where edges[i] = [ai, bi] indicates that there is an edge between ai and bi in the graph. Return the number of connected components in the graph.

You are given an m x n binary matrix grid. An island is a group of 1's (representing land) connected 4-directionally (horizontal or vertical). Return the maximum area of an island in grid. If there is no island, return 0.

You are given an m x n grid where each cell can have one of three values: 0 (empty), 1 (fresh orange), or 2 (rotten orange). Every minute, any fresh orange that is 4-directionally adjacent to a rotten orange becomes rotten. Return the minimum number of minutes until no cell has a fresh orange. If impossible, return -1.

There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take course bi first if you want to take course ai. Return the ordering of courses you should take to finish all courses. If there are many valid answers, return any of them. If it is impossible to finish all courses, return an empty array.

Given an m x n matrix board containing 'X' and 'O', capture all regions that are 4-directionally surrounded by 'X'. A region is captured by flipping all 'O's into 'X's in that surrounded region.

Given two words beginWord and endWord, and a dictionary wordList, return the number of words in the shortest transformation sequence from beginWord to endWord (changing one letter at a time).

There is a new alien language that uses the English alphabet. However, the order among the letters is unknown to you. You are given a list of strings words from the alien language's dictionary, where the strings in words are sorted lexicographically by the rules of this new language. Return a string of the unique letters in the new alien language sorted in lexicographically increasing order by the new rules. If there is no solution, return "".

You are given a network of n nodes, labeled from 1 to n. You are also given times, a list of travel times as directed edges times[i] = (ui, vi, wi), where ui is the source node, vi is the target node, and wi is the time it takes for a signal to travel from source to target. We send a signal from a given node k. Return the minimum time it takes for all the n nodes to receive the signal. If it is impossible for all the n nodes to receive the signal, return -1.

There is an undirected graph with n nodes, where each node is numbered between 0 and n - 1. You are given a 2D array graph, where graph[u] is an array of nodes that node u is adjacent to. Return true if and only if it is bipartite.

There are n cities. Some of them are connected, while some are not. If city a is connected directly with city b, and city b is connected directly with city c, then city a is connected indirectly with city c. A province is a group of directly or indirectly connected cities and no other cities outside of the group. Return the total number of provinces.

Given an n x n binary matrix grid, return the length of the shortest clear path in the matrix. If there is no clear path, return -1. A clear path is a path from the top-left cell (0, 0) to the bottom-right cell (n-1, n-1) such that all the visited cells are 0. You may move 8-directionally (horizontal, vertical, and diagonal).

You have a graph of n nodes. You are given an integer n and an array edges where edges[i] = [ai, bi] indicates that there is an edge between ai and bi in the graph. Return the number of connected components in the graph.

N couples sit in 2N seats arranged in a row. You want every couple to sit side by side. Couples are numbered (0,1), (2,3), etc. A swap consists of choosing any two people and having them swap seats. Return the minimum number of swaps so that every couple is sitting side by side.

Given a m x n grid where -1 is a wall, 0 is a gate, and INF (2147483647) is an empty room. Fill each empty room with the distance to its nearest gate. If impossible to reach a gate, leave it as INF.

Given an integer array nums, find the subarray with the largest sum, and return its sum.

You are given an integer array nums. You are initially positioned at the array's first index, and each element in the array represents your maximum jump length at that position. Return true if you can reach the last index, or false otherwise.

Given a string s containing only three types of characters: '(', ')' and '*', return true if s is valid.

You are given a string s. We want to partition the string into as many parts as possible so that each letter appears in at most one part. Return a list of integers representing the size of these parts.

Alice has some number of cards and she wants to rearrange the cards into groups so that each group is of size groupSize, and consists of groupSize consecutive cards. Given an integer array hand where hand[i] is the value written on the ith card and an integer groupSize, return true if she can rearrange the cards, or false otherwise.

There are n gas stations along a circular route, where the amount of gas at the ith station is gas[i]. You have a car with an unlimited gas tank and it costs cost[i] of gas to travel from the ith station to its next (i + 1)th station. You begin the journey with an empty tank at one of the gas stations. Return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1.

You are given a 0-indexed array of integers nums of length n. You are initially positioned at nums[0]. Each element nums[i] represents the maximum length of a forward jump from index i. Return the minimum number of jumps to reach nums[n - 1].

There are some spherical balloons taped onto a flat wall that represents the XY-plane. The balloons are represented as a 2D integer array points where points[i] = [xstart, xend] denotes a balloon whose horizontal diameter stretches between xstart and xend. You need to find the minimum number of arrows that must be shot to burst all balloons. An arrow can be shot at any integer x coordinate. A balloon with xstart and xend is burst by an arrow shot at x if xstart <= x <= xend.

There are n children standing in a line. Each child is assigned a rating value given in the integer array ratings. You are giving candies to these children subjected to the following requirements: Each child must have at least one candy. Children with a higher rating get more candies than their neighbors. Return the minimum number of candies you need to have to distribute the candies to the children.

You are given an array of people, people, which are the attributes of some people in a queue (not necessarily in order). Each people[i] = [hi, ki] represents the ith person of height hi with exactly ki other people in front who have a height >= hi. Reconstruct and return the queue that is represented by the input array.

The median is the middle value in an ordered integer list. Implement the MedianFinder class.

Given a characters array tasks, representing the tasks a CPU needs to do, where each letter represents a different task. Tasks could be done in any order. Each task is done in one unit of time. For each unit of time, the CPU could complete either one task or just be idle. However, there is a non-negative integer n that represents the cooldown period between two same tasks (the same letter in the array), that is that there must be at least n units of time between any two same tasks.

Design a class to find the kth largest element in a stream. Note that it is the kth largest element in the sorted order, not the kth distinct element.

Given an array of points where points[i] = [xi, yi] represents a point on the X-Y plane and an integer k, return the k closest points to the origin (0, 0). The distance between two points is the Euclidean distance. You may return the answer in any order.

Given an integer array nums and an integer k, return the kth largest element in the array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

Given two sorted arrays nums1 and nums2, find k pairs (u1, v1), (u2, v2), ..., (uk, vk) with the smallest sums.

Design a simplified version of Twitter where users can post tweets, follow/unfollow another user, and see the 10 most recent tweets in their news feed. Implement the Twitter class with postTweet, getNewsFeed, follow, and unfollow methods.

A city's skyline is the outer contour of the silhouette formed by all the buildings in that city when viewed from a distance. Given the locations and heights of all the buildings, return the skyline formed by these buildings collectively.

Given an array of meeting time intervals consisting of start and end times [[s1,e1],[s2,e2],...] (si < ei), determine if a person could attend all meetings.

Given an array of meeting time intervals consisting of start and end times [[s1,e1],[s2,e2],...] (si < ei), find the minimum number of conference rooms required.

Given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals, and return an array of the non-overlapping intervals that cover all the intervals in the input.

Given the head of a singly linked list, reverse the list, and return the reversed list.

Given head, the head of a linked list, determine if the linked list has a cycle in it. A cycle exists if there is some node that can be reached again by continuously following the next pointer.

You are given the head of a singly linked-list. The list can be represented as: L0 → L1 → … → Ln - 1 → Ln. Reorder the list to be on the following form: L0 → Ln → L1 → Ln - 1 → L2 → Ln - 2 → …

You are given the heads of two sorted linked lists list1 and list2. Merge the two lists into one sorted list. The list should be made by splicing together the nodes of the first two lists.

Given the head of a linked list, remove the nth node from the end of the list and return its head.

You are given an array of k linked-lists lists, each linked-list is sorted in ascending order. Merge all the linked-lists into one sorted linked-list and return it.

Given two non-empty linked lists representing two non-negative integers stored in reverse order, add them and return the sum as a linked list.

A linked list of length n is given such that each node contains an additional random pointer, which could point to any node in the list, or null. Construct a deep copy of the list.

Given the head of a linked list, reverse the nodes of the list k at a time, and return the modified list.

Given the head of a singly linked list, return true if it is a palindrome or false otherwise.

Given a linked list, swap every two adjacent nodes and return its head. You must solve the problem without modifying the values in the nodes.

Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null.

Given the heads of two singly linked-lists headA and headB, return the node at which the two lists intersect. If the two linked lists have no intersection at all, return null.

Given the head of a linked list, rotate the list to the right by k places.

Given the head of a singly linked list, group all the nodes with odd indices together followed by the nodes with even indices, and return the reordered list. The first node is considered odd, and the second node is even, and so on.

Sort a linked list in O(n log n) time and O(1) memory space using merge sort.

Given an integer n, return the number of prime numbers that are strictly less than n.

Given a string columnTitle that represents the column title as appears in an Excel sheet, return its corresponding column number.

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string. You must not use any built-in BigInteger library or convert the inputs to integer directly.

You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise). You have to rotate the image in-place.

Given an m x n matrix, return all elements of the matrix in spiral order.

Given an m x n integer matrix matrix, if an element is 0, set its entire row and column to 0s. You must do it in place.

Given a positive integer n, generate an n x n matrix filled with elements from 1 to n² in spiral order.

You are given an array prices where prices[i] is the price of a given stock on the ith day. You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.

Given a string s, find the length of the longest substring without repeating characters.

Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise. In other words, return true if one of s1's permutations is the substring of s2.

You are given an array of integers nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You can only see the k numbers in the window. Each time the sliding window moves right by one position. Return the max sliding window.

Given two strings s and t of lengths m and n respectively, return the minimum window substring of s such that every character in t (including duplicates) is included in the window. If there is no such substring, return the empty string "".

Given a string s containing just the characters "(", ")", "{", "}", "[" and "]", determine if the input string is valid. An input string is valid if: Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order.

Given an array of integers temperatures represents the daily temperatures, return an array answer such that answer[i] is the number of days you have to wait after the ith day to get a warmer temperature. If there is no future day for which this is possible, keep answer[i] == 0.

Evaluate the value of an arithmetic expression in Reverse Polish Notation. Valid operators are +, -, *, and /. Each operand may be an integer or another expression. Note that division between two integers should truncate toward zero.

Given an array of integers heights representing the histogram's bar height where the width of each bar is 1, return the area of the largest rectangle in the histogram.

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

Given string num representing a non-negative integer num, and an integer k, return the smallest possible integer after removing k digits from num.

Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation. The expression string may contain open ( and closing ) parentheses, the plus + or minus - sign, non-negative integers and empty spaces.

Write a function to find the longest common prefix string amongst an array of strings.

Given a roman numeral, convert it to an integer.

Given two binary strings a and b, return their sum as a binary string.

Given two strings needle and haystack, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Given the root of a binary tree, invert the tree, and return its root.

Given the roots of two binary trees root and subRoot, return true if there is a subtree of root with the same structure and node values of subRoot and false otherwise.

Find the lowest common ancestor (LCA) of two nodes in a binary tree. The LCA is the lowest node that has both p and q as descendants.

Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

Given the root of a binary search tree, and an integer k, return the kth smallest value (1-indexed) of all the values of the nodes in the tree.

Given two integer arrays preorder and inorder where preorder is the preorder traversal of a binary tree and inorder is the inorder traversal of the same tree, construct and return the binary tree.

Design an algorithm to serialize and deserialize a binary tree. There is no restriction on how your serialization/deserialization algorithm should work.

A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence at most once. Note that the path does not need to pass through the root. The path sum is the sum of the node's values in the path. Given the root of a binary tree, return the maximum path sum of any non-empty path.

Given a binary tree root, a node X in the tree is named good if in the path from root to X there are no nodes with a value greater than X. Return the number of good nodes in the binary tree.

Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

Given the root of a binary tree and an integer targetSum, return true if the tree has a root-to-leaf path such that adding up all the values along the path equals targetSum.

Given the root of a binary tree, flatten the tree into a "linked list": The "linked list" should use the same TreeNode class where the right child pointer points to the next node in the list and the left child pointer is always null. The "linked list" should be in the same order as a pre-order traversal of the binary tree.

You are given a perfect binary tree where all leaves are on the same level, and every parent has two children. Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL.

Given the root of a binary tree, return the zigzag level order traversal of its nodes values. (i.e., from left to right, then right to left for the next level and alternate between).

Given the root of a binary tree, return the maximum width of the given tree. The maximum width of a tree is the maximum width among all levels. The width of one level is defined as the length between the end-nodes (the leftmost and rightmost non-null nodes), where the null nodes between the end-nodes that would be present in a complete binary tree are also counted.

Given an integer array nums where the elements are sorted in ascending order, convert it to a height-balanced binary search tree.

Two nodes of a BST are swapped by mistake. Recover the tree without changing its structure.

Implement the BSTIterator class that represents an iterator over the in-order traversal of a binary search tree (BST). The class should have a constructor that takes the root of a BST, a next() method that returns the next smallest number, and a hasNext() method that returns whether there is a next number.

A trie (prefix tree) is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. Implement the Trie class with insert, search, and startsWith methods.

Design a data structure that supports adding new words and finding if a string matches any previously added string. Implement the WordDictionary class with addWord(word) and search(word) methods. search(word) can search a literal word or a regular expression string containing only letters a-z or '.'. A '.' means it can represent any one letter.

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward.

Find all unique triplets in the array which gives the sum of zero.

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining.

Write a function that reverses a string. The input string is given as an array of characters s. You must do this by modifying the input array in-place with O(1) extra memory.

Given an integer array nums sorted in non-decreasing order, remove the duplicates in-place such that each unique element appears only once. Return the number of unique elements.

Given an array with n objects colored red, white, or blue (represented by 0, 1, and 2), sort them in-place so that objects of the same color are adjacent (Dutch National Flag problem).