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

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].

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

Design an algorithm to encode a list of strings to a string. The encoded string is then sent over the network and is decoded back to the original list of strings.

Given an array of integers nums and an integer k, return the total number of subarrays whose sum equals to k.

Given an integer array nums, move all 0's to the end of it while maintaining the relative order of the non-zero elements. Note that you must do this in-place without making a copy of 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.

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 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 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.

Given a string s that contains parentheses and letters, remove the minimum number of invalid parentheses to make the input string valid. Return all possible results.

Given a string num that contains only digits and an integer target, return all possible expressions by adding the binary operators +, -, or * between the digits so that the expression evaluates to the target value.

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.

Koko loves to eat bananas. There are n piles of bananas, the ith pile has piles[i] bananas. The guards have gone and will come back in h hours. Koko can decide her bananas-per-hour eating speed of k. Each hour, she chooses some pile of bananas and eats k bananas from that pile. If the pile has less than k bananas, she eats all of them instead and will not eat any more bananas during this hour. Return the minimum integer k such that she can eat all the bananas within h hours.

Given an n x n matrix where each of the rows and columns is sorted in ascending order, return the kth smallest element in the matrix. Note that it is the kth smallest element in the sorted order, not the kth distinct element.

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.

Given an integer array nums and an integer m, split the array into m non-empty continuous subarrays. Write an algorithm to minimize the largest sum among these m subarrays.

Given a sorted array, find the k closest integers to x. The result should also be sorted in ascending order.

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].

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.

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 an in-memory file system to simulate ls, mkdir, addContentToFile, and readContentFromFile operations.

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 a string s, return the number of palindromic substrings in it. A string is a palindrome when it reads the same backward as forward.

You are given an integer array nums and an integer target. You want to build an expression out of nums by adding one of the symbols '+' and '-' before each integer in nums. Return the number of different expressions that you can build, which evaluates to target.

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

Given an m x n binary matrix filled with 0's and 1's, find the largest square containing only 1's and return its area.

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).

You are given n balloons, indexed from 0 to n - 1. Each balloon is painted with a number on it represented by an array nums. You are asked to burst all the balloons. If you burst the ith balloon, you will get nums[i - 1] * nums[i] * nums[i + 1] coins. If i - 1 or i + 1 goes out of bounds, treat it as if there is a balloon with a 1 painted on it. Return the maximum coins you can collect by bursting the balloons wisely.

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.

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.

Return a list of grid coordinates where water can flow to both the Pacific and Atlantic oceans.

Given n nodes labeled from 0 to n - 1 and a list of undirected edges, write a function to check whether these edges make up a valid tree.

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.

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).

Given a list of accounts where each account contains a name and emails, merge accounts belonging to the same person (accounts that share at least one common email).

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 an array of variable pairs equations and an array of real numbers values, where equations[i] = [Ai, Bi] and values[i] represent the equation Ai / Bi = values[i]. Each Ai or Bi is a string that represents a single variable. You are also given some queries, where queries[j] = [Cj, Dj] represents the jth query where you must find the answer for Cj / Dj = ?. Return the answers to all queries.

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.

There are n servers numbered from 0 to n - 1 connected by undirected server-to-server connections forming a network where connections[i] = [ai, bi] represents a connection between servers ai and bi. Any server can reach other servers directly or indirectly through the network. A critical connection is a connection that, if removed, will make some servers unable to reach some other server. Return all critical connections in the network in any order.

A transformation sequence from word beginWord to word endWord using a dictionary wordList is a sequence of words beginWord -> s1 -> s2 -> ... -> sk such that every adjacent pair differs by a single letter, every si is in wordList, and sk == endWord. Given two words and a dictionary, return all the shortest transformation sequences.

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 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.

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

Given a string s, rearrange the characters of s so that any two adjacent characters are not the same. Return any possible rearrangement of s or return "" if not possible.

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.

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.

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 intervals intervals where intervals[i] = [starti, endi], return the minimum number of intervals you need to remove to make the rest of the intervals non-overlapping.

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.

You are given an array of non-overlapping intervals intervals where intervals[i] = [starti, endi] represent the start and the end of the ith interval and intervals is sorted in ascending order by starti. You are also given an interval newInterval = [start, end] that represents the start and end of another interval. Insert newInterval into intervals such that intervals is still sorted in ascending order by starti and intervals still does not have any overlapping intervals (merge overlapping intervals if necessary).

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.

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.

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.

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

Implement pow(x, n), which calculates x raised to the power n (i.e., x^n). Use the fast exponentiation algorithm to achieve O(log n) time complexity.

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

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.

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 p, return an array of all the start indices of p's anagrams in s. You may return the answer in any order.

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 and an integer k, return the length of the longest substring of s that contains at most k distinct characters.

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.

Given an array of asteroids, find out the state after all collisions. Positive means moving right, negative means moving left. The larger asteroid survives.

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.

Given an encoded string, return its decoded string. The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times.

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 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.

Given a binary search tree (BST), find the lowest common ancestor (LCA) node of two given nodes in the BST.

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 the root of a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

Given the root of a binary tree, return the length of the diameter of the tree.

Given a binary tree, determine if it is height-balanced. A height-balanced binary tree is a binary tree in which the depth of the two subtrees of every node never differs by more than one.

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 all root-to-leaf paths in any order.

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 two numbers that add up to a specific target number in a sorted array. Return 1-based indices.

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

Find two lines that together with the x-axis form a container, such that the container contains the most water.

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.

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).

Given a string s, return true if the s can be palindrome after deleting at most one character from it.

Given an integer array nums sorted in non-decreasing order, return an array of the squares of each number sorted in non-decreasing order.