Datasets:

kreimben

/

leetcode_questions

like 0

Symmetric Tree

symmetric-tree

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

Tree,Depth-First Search,Breadth-First Search,Binary Tree

Easy

Binary Tree Level Order Traversal

binary-tree-level-order-traversal

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).

Tree,Breadth-First Search,Binary Tree

Medium

103,107,111,314,637,764,1035

Binary Tree Zigzag Level Order Traversal

binary-tree-zigzag-level-order-traversal

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).

Tree,Breadth-First Search,Binary Tree

Medium

102

Maximum Depth of Binary Tree

maximum-depth-of-binary-tree

Given the root of a binary tree, return its maximum depth. A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Tree,Depth-First Search,Breadth-First Search,Binary Tree

Easy

110,111,774,1492

Construct Binary Tree from Preorder and Inorder Traversal

construct-binary-tree-from-preorder-and-inorder-traversal

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.

Array,Hash Table,Divide and Conquer,Tree,Binary Tree

Medium

106

Construct Binary Tree from Inorder and Postorder Traversal

construct-binary-tree-from-inorder-and-postorder-traversal

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

Array,Hash Table,Divide and Conquer,Tree,Binary Tree

Medium

105

Binary Tree Level Order Traversal II

binary-tree-level-order-traversal-ii

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

Tree,Breadth-First Search,Binary Tree

Medium

102,637

Convert Sorted Array to Binary Search Tree

convert-sorted-array-to-binary-search-tree

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

Array,Divide and Conquer,Tree,Binary Search Tree,Binary Tree

Easy

109

Convert Sorted List to Binary Search Tree

convert-sorted-list-to-binary-search-tree

Given the head of a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST. For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Linked List,Divide and Conquer,Tree,Binary Search Tree,Binary Tree

Medium

108,2306

Balanced Binary Tree

balanced-binary-tree

Given a binary tree, determine if it is height-balanced. For this problem, a height-balanced binary tree is defined as: a binary tree in which the left and right subtrees of every node differ in height by no more than 1.

Tree,Depth-First Search,Binary Tree

Easy

104

Minimum Depth of Binary Tree

minimum-depth-of-binary-tree

Given a binary tree, find its minimum depth. The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node. Note: A leaf is a node with no children.

Tree,Depth-First Search,Breadth-First Search,Binary Tree

Easy

102,104

Path Sum

path-sum

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. A leaf is a node with no children.

Tree,Depth-First Search,Breadth-First Search,Binary Tree

Easy

113,124,129,437,666

Path Sum II

path-sum-ii

Given the root of a binary tree and an integer targetSum, return all root-to-leaf paths where the sum of the node values in the path equals targetSum. Each path should be returned as a list of the node values, not node references. A root-to-leaf path is a path starting from the root and ending at any leaf node. A leaf is a node with no children.

Backtracking,Tree,Depth-First Search,Binary Tree

Medium

112,257,437,666,2217

Flatten Binary Tree to Linked List

flatten-binary-tree-to-linked-list

Given the root of a binary tree, flatten the tree into a "linked list":

Linked List,Stack,Tree,Depth-First Search,Binary Tree

Medium

If you notice carefully in the flattened tree, each node's right child points to the next node of a pre-order traversal.

766,1796

Distinct Subsequences

distinct-subsequences

Given two strings s and t, return the number of distinct subsequences of s which equals t. A string's subsequence is a new string formed from the original string by deleting some (can be none) of the characters without disturbing the remaining characters' relative positions. (i.e., "ACE" is a subsequence of "ABCDE" while "AEC" is not). The test cases are generated so that the answer fits on a 32-bit signed integer.

String,Dynamic Programming

Hard

2115

Populating Next Right Pointers in Each Node

populating-next-right-pointers-in-each-node

You are given a perfect binary tree where all leaves are on the same level, and every parent has two children. The binary tree has the following definition: 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. Initially, all next pointers are set to NULL.

Linked List,Tree,Depth-First Search,Breadth-First Search,Binary Tree

Medium

117,199

Populating Next Right Pointers in Each Node II

populating-next-right-pointers-in-each-node-ii

Given a binary tree 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. Initially, all next pointers are set to NULL.

Linked List,Tree,Depth-First Search,Breadth-First Search,Binary Tree

Medium

116

Pascal's Triangle

pascals-triangle

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 as shown:

Array,Dynamic Programming

Easy

119

Pascal's Triangle II

pascals-triangle-ii

Given an integer rowIndex, return the rowIndexth (0-indexed) row of the Pascal's triangle. In Pascal's triangle, each number is the sum of the two numbers directly above it as shown:

Array,Dynamic Programming

Easy

118,2324

Triangle

triangle

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.

Array,Dynamic Programming

Medium

Best Time to Buy and Sell Stock

best-time-to-buy-and-sell-stock

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. Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

Array,Dynamic Programming

Easy

53,122,123,188,309,2138,2144

Best Time to Buy and Sell Stock II

best-time-to-buy-and-sell-stock-ii

You are given an integer array prices where prices[i] is the price of a given stock on the ith day. On each day, you may decide to buy and/or sell the stock. You can only hold at most one share of the stock at any time. However, you can buy it then immediately sell it on the same day. Find and return the maximum profit you can achieve.

Array,Dynamic Programming,Greedy

Medium

121,123,188,309,714

Best Time to Buy and Sell Stock III

best-time-to-buy-and-sell-stock-iii

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

Array,Dynamic Programming

Hard

121,122,188,689

Binary Tree Maximum Path Sum

binary-tree-maximum-path-sum

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 of a path 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.

Dynamic Programming,Tree,Depth-First Search,Binary Tree

Hard

112,129,666,687,1492

Valid Palindrome

valid-palindrome

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. Alphanumeric characters include letters and numbers. Given a string s, return true if it is a palindrome, or false otherwise.

Two Pointers,String

Easy

234,680,2130,2231

Word Ladder II

word-ladder-ii

A transformation sequence from word beginWord to word endWord using a dictionary wordList is a sequence of words beginWord -> s1 -> s2 -> ... -> sk such that: Given two words, beginWord and endWord, and a dictionary wordList, return all the shortest transformation sequences from beginWord to endWord, or an empty list if no such sequence exists. Each sequence should be returned as a list of the words [beginWord, s1, s2, ..., sk].

Hash Table,String,Backtracking,Breadth-First Search

Hard

127,2276

Word Ladder

word-ladder

A transformation sequence from word beginWord to word endWord using a dictionary wordList is a sequence of words beginWord -> s1 -> s2 -> ... -> sk such that: Given two words, beginWord and endWord, and a dictionary wordList, return the number of words in the shortest transformation sequence from beginWord to endWord, or 0 if no such sequence exists.

Hash Table,String,Breadth-First Search

Hard

126,433

Longest Consecutive Sequence

longest-consecutive-sequence

Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence. You must write an algorithm that runs in O(n) time.

Array,Hash Table,Union Find

Medium

298,2278

Sum Root to Leaf Numbers

sum-root-to-leaf-numbers

You are given the root of a binary tree containing digits from 0 to 9 only. Each root-to-leaf path in the tree represents a number. Return the total sum of all root-to-leaf numbers. Test cases are generated so that the answer will fit in a 32-bit integer. A leaf node is a node with no children.

Tree,Depth-First Search,Binary Tree

Medium

112,124,1030

Surrounded Regions

surrounded-regions

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.

Array,Depth-First Search,Breadth-First Search,Union Find,Matrix

Medium

200,286

Palindrome Partitioning

palindrome-partitioning

Given a string s, partition s such that every substring of the partition is a palindrome. Return all possible palindrome partitioning of s. A palindrome string is a string that reads the same backward as forward.

String,Dynamic Programming,Backtracking

Medium

132,1871

Palindrome Partitioning II

palindrome-partitioning-ii

Given a string s, partition s such that every substring of the partition is a palindrome. Return the minimum cuts needed for a palindrome partitioning of s.

String,Dynamic Programming

Hard

131,1871

Clone Graph

clone-graph

Given a reference of a node in a connected undirected graph. Return a deep copy (clone) of the graph. Each node in the graph contains a value (int) and a list (List[Node]) of its neighbors. Test case format: For simplicity, each node's value is the same as the node's index (1-indexed). For example, the first node with val == 1, the second node with val == 2, and so on. The graph is represented in the test case using an adjacency list. An adjacency list is a collection of unordered lists used to represent a finite graph. Each list describes the set of neighbors of a node in the graph. The given node will always be the first node with val = 1. You must return the copy of the given node as a reference to the cloned graph.

Hash Table,Depth-First Search,Breadth-First Search,Graph

Medium

138,1624,1634

Gas Station

gas-station

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. Given two integer arrays gas and cost, return the starting gas station's index if you can travel around the circuit once in the clockwise direction, otherwise return -1. If there exists a solution, it is guaranteed to be unique

Array,Greedy

Medium

1346

Candy

candy

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: Return the minimum number of candies you need to have to distribute the candies to the children.

Array,Greedy

Hard

Single Number

single-number

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 a linear runtime complexity and use only constant extra space.

Array,Bit Manipulation

Easy

137,260,268,287,389

Single Number II

single-number-ii

Given an integer array nums where every element appears three times except for one, which appears exactly once. Find the single element and return it. You must implement a solution with a linear runtime complexity and use only constant extra space.

Array,Bit Manipulation

Medium

136,260

Copy List with Random Pointer

copy-list-with-random-pointer

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. The deep copy should consist of exactly n brand new nodes, where each new node has its value set to the value of its corresponding original node. Both the next and random pointer of the new nodes should point to new nodes in the copied list such that the pointers in the original list and copied list represent the same list state. None of the pointers in the new list should point to nodes in the original list. For example, if there are two nodes X and Y in the original list, where X.random --> Y, then for the corresponding two nodes x and y in the copied list, x.random --> y. Return the head of the copied linked list. The linked list is represented in the input/output as a list of n nodes. Each node is represented as a pair of [val, random_index] where: Your code will only be given the head of the original linked list.

Hash Table,Linked List

Medium

Just iterate the linked list and create copies of the nodes on the go. Since a node can be referenced from multiple nodes due to the random pointers, make sure you are not making multiple copies of the same node. You may want to use extra space to keep old node ---> new node mapping to prevent creating multiples copies of same node. We can avoid using extra space for old node ---> new node mapping, by tweaking the original linked list. Simply interweave the nodes of the old and copied list. For e.g. Old List: A --> B --> C --> D InterWeaved List: A --> A' --> B --> B' --> C --> C' --> D --> D' The interweaving is done using next pointers and we can make use of interweaved structure to get the correct reference nodes for random pointers.

133,1624,1634

Word Break

word-break

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. Note that the same word in the dictionary may be reused multiple times in the segmentation.

Hash Table,String,Dynamic Programming,Trie,Memoization

Medium

140

Word Break II

word-break-ii

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. Note that the same word in the dictionary may be reused multiple times in the segmentation.

Hash Table,String,Dynamic Programming,Backtracking,Trie,Memoization

Hard

139,472

Linked List Cycle

linked-list-cycle

Given head, the head of a linked list, determine if the linked list has a cycle in it. There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter. Return true if there is a cycle in the linked list. Otherwise, return false.

Hash Table,Linked List,Two Pointers

Easy

142,202

Linked List Cycle II

linked-list-cycle-ii

Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null. There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to (0-indexed). It is -1 if there is no cycle. Note that pos is not passed as a parameter. Do not modify the linked list.

Hash Table,Linked List,Two Pointers

Medium

141,287

Reorder List

reorder-list

You are given the head of a singly linked-list. The list can be represented as: Reorder the list to be on the following form: You may not modify the values in the list's nodes. Only nodes themselves may be changed.

Linked List,Two Pointers,Stack,Recursion

Medium

2216

Binary Tree Preorder Traversal

binary-tree-preorder-traversal

Given the root of a binary tree, return the preorder traversal of its nodes' values.

Stack,Tree,Depth-First Search,Binary Tree

Easy

94,255,775

Binary Tree Postorder Traversal

binary-tree-postorder-traversal

Given the root of a binary tree, return the postorder traversal of its nodes' values.

Stack,Tree,Depth-First Search,Binary Tree

Easy

94,776

LRU Cache

lru-cache

Design a data structure that follows the constraints of a Least Recently Used (LRU) cache. Implement the LRUCache class: The functions get and put must each run in O(1) average time complexity.

Hash Table,Linked List,Design,Doubly-Linked List

Medium

460,588,604,1903

Insertion Sort List

insertion-sort-list

Given the head of a singly linked list, sort the list using insertion sort, and return the sorted list's head. The steps of the insertion sort algorithm: The following is a graphical example of the insertion sort algorithm. The partially sorted list (black) initially contains only the first element in the list. One element (red) is removed from the input data and inserted in-place into the sorted list with each iteration.

Linked List,Sorting

Medium

148,850

Sort List

sort-list

Given the head of a linked list, return the list after sorting it in ascending order.

Linked List,Two Pointers,Divide and Conquer,Sorting,Merge Sort

Medium

21,75,147,1992

Max Points on a Line

max-points-on-a-line

Given an array of points where points[i] = [xi, yi] represents a point on the X-Y plane, return the maximum number of points that lie on the same straight line.

Array,Hash Table,Math,Geometry

Hard

356,2287

Evaluate Reverse Polish Notation

evaluate-reverse-polish-notation

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. It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.

Array,Math,Stack

Medium

224,282

Reverse Words in a String

reverse-words-in-a-string

Given an input string s, reverse the order of the words. A word is defined as a sequence of non-space characters. The words in s will be separated by at least one space. Return a string of the words in reverse order concatenated by a single space. Note that s may contain leading or trailing spaces or multiple spaces between two words. The returned string should only have a single space separating the words. Do not include any extra spaces.

Two Pointers,String

Medium

186

Maximum Product Subarray

maximum-product-subarray

Given an integer array nums, find a contiguous non-empty subarray within the array that has the largest product, and return the product. The test cases are generated so that the answer will fit in a 32-bit integer. A subarray is a contiguous subsequence of the array.

Array,Dynamic Programming

Medium

53,198,238,628,713

Find Minimum in Rotated Sorted Array

find-minimum-in-rotated-sorted-array

Suppose an array of length n sorted in ascending order is rotated between 1 and n times. For example, the array nums = [0,1,2,4,5,6,7] might become: Notice that rotating an array [a[0], a[1], a[2], ..., a[n-1]] 1 time results in the array [a[n-1], a[0], a[1], a[2], ..., a[n-2]]. Given the sorted rotated array nums of unique elements, return the minimum element of this array. You must write an algorithm that runs in O(log n) time.

Array,Binary Search

Medium

Array was originally in ascending order. Now that the array is rotated, there would be a point in the array where there is a small deflection from the increasing sequence. eg. The array would be something like [4, 5, 6, 7, 0, 1, 2]. You can divide the search space into two and see which direction to go. Can you think of an algorithm which has O(logN) search complexity? All the elements to the left of inflection point > first element of the array. All the elements to the right of inflection point < first element of the array.

33,154

Find Minimum in Rotated Sorted Array II

find-minimum-in-rotated-sorted-array-ii

Suppose an array of length n sorted in ascending order is rotated between 1 and n times. For example, the array nums = [0,1,4,4,5,6,7] might become: Notice that rotating an array [a[0], a[1], a[2], ..., a[n-1]] 1 time results in the array [a[n-1], a[0], a[1], a[2], ..., a[n-2]]. Given the sorted rotated array nums that may contain duplicates, return the minimum element of this array. You must decrease the overall operation steps as much as possible.

Array,Binary Search

Hard

153

Min Stack

min-stack

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

Stack,Design

Easy

Consider each node in the stack having a minimum value. (Credits to @aakarshmadhavan)

239,716

Binary Tree Upside Down

binary-tree-upside-down

Tree,Depth-First Search,Binary Tree

Medium

206

Read N Characters Given Read4

read-n-characters-given-read4

String,Simulation,Interactive

Easy

158

Read N Characters Given read4 II - Call Multiple Times

read-n-characters-given-read4-ii-call-multiple-times

String,Simulation,Interactive

Hard

157

Longest Substring with At Most Two Distinct Characters

longest-substring-with-at-most-two-distinct-characters

Hash Table,String,Sliding Window

Medium

3,239,340,1034

Intersection of Two Linked Lists

intersection-of-two-linked-lists

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. For example, the following two linked lists begin to intersect at node c1: The test cases are generated such that there are no cycles anywhere in the entire linked structure. Note that the linked lists must retain their original structure after the function returns. Custom Judge: The inputs to the judge are given as follows (your program is not given these inputs): The judge will then create the linked structure based on these inputs and pass the two heads, headA and headB to your program. If you correctly return the intersected node, then your solution will be accepted.

Hash Table,Linked List,Two Pointers

Easy

599

One Edit Distance

one-edit-distance

Two Pointers,String

Medium

72

Find Peak Element

find-peak-element

A peak element is an element that is strictly greater than its neighbors. Given an 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 may imagine that nums[-1] = nums[n] = -∞. You must write an algorithm that runs in O(log n) time.

Array,Binary Search

Medium

882,2047,2273,2316

Missing Ranges

missing-ranges

Array

Easy

228

Maximum Gap

maximum-gap

Given an integer array nums, return the maximum difference between two successive elements in its sorted form. If the array contains less than two elements, return 0. You must write an algorithm that runs in linear time and uses linear extra space.

Array,Sorting,Bucket Sort,Radix Sort

Hard

Compare Version Numbers

compare-version-numbers

Given two version numbers, version1 and version2, compare them. Version numbers consist of one or more revisions joined by a dot '.'. Each revision consists of digits and may contain leading zeros. Every revision contains at least one character. Revisions are 0-indexed from left to right, with the leftmost revision being revision 0, the next revision being revision 1, and so on. For example 2.5.33 and 0.1 are valid version numbers. To compare version numbers, compare their revisions in left-to-right order. Revisions are compared using their integer value ignoring any leading zeros. This means that revisions 1 and 001 are considered equal. If a version number does not specify a revision at an index, then treat the revision as 0. For example, version 1.0 is less than version 1.1 because their revision 0s are the same, but their revision 1s are 0 and 1 respectively, and 0 < 1. Return the following:

Two Pointers,String

Medium

Fraction to Recurring Decimal

fraction-to-recurring-decimal

Given two integers representing the numerator and denominator of a fraction, return the fraction in string format. If the fractional part is repeating, enclose the repeating part in parentheses. If multiple answers are possible, return any of them. It is guaranteed that the length of the answer string is less than 104 for all the given inputs.

Hash Table,Math,String

Medium

No scary math, just apply elementary math knowledge. Still remember how to perform a long division? Try a long division on 4/9, the repeating part is obvious. Now try 4/333. Do you see a pattern? Notice that once the remainder starts repeating, so does the divided result. Be wary of edge cases! List out as many test cases as you can think of and test your code thoroughly.

Two Sum II - Input Array Is Sorted

two-sum-ii-input-array-is-sorted

Given a 1-indexed array of integers numbers that is already sorted in non-decreasing order, find two numbers such that they add up to a specific target number. Let these two numbers be numbers[index1] and numbers[index2] where 1 <= index1 < index2 <= numbers.length. Return the indices of the two numbers, index1 and index2, added by one as an integer array [index1, index2] of length 2. The tests are generated such that there is exactly one solution. You may not use the same element twice. Your solution must use only constant extra space.

Array,Two Pointers,Binary Search

Medium

1,653,1083

Excel Sheet Column Title

excel-sheet-column-title

Given an integer columnNumber, return its corresponding column title as it appears in an Excel sheet. For example:

Math,String

Easy

171,2304

Majority Element

majority-element

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.

Array,Hash Table,Divide and Conquer,Sorting,Counting

Easy

229,1102

Two Sum III - Data structure design

two-sum-iii-data-structure-design

Array,Hash Table,Two Pointers,Design,Data Stream

Easy

1,288,653

Excel Sheet Column Number

excel-sheet-column-number

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

Math,String

Easy

168,2304

Factorial Trailing Zeroes

factorial-trailing-zeroes

Given an integer n, return the number of trailing zeroes in n!. Note that n! = n * (n - 1) * (n - 2) * ... * 3 * 2 * 1.

Math

Medium

233,809,2222

Binary Search Tree Iterator

binary-search-tree-iterator

Implement the BSTIterator class that represents an iterator over the in-order traversal of a binary search tree (BST): Notice that by initializing the pointer to a non-existent smallest number, the first call to next() will return the smallest element in the BST. You may assume that next() calls will always be valid. That is, there will be at least a next number in the in-order traversal when next() is called.

Stack,Tree,Design,Binary Search Tree,Binary Tree,Iterator

Medium

94,251,281,284,285,1729

Dungeon Game

dungeon-game

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 laid out in a 2D grid. Our valiant knight was initially positioned in the top-left room and must fight his way through dungeon to rescue the princess. The knight has an initial health point represented by a positive integer. If at any point his health point drops to 0 or below, he dies immediately. Some of the rooms are guarded by demons (represented by negative integers), so the knight loses health upon entering these rooms; other rooms are either empty (represented as 0) or contain magic orbs that increase the knight's health (represented by positive integers). To reach the princess as quickly as possible, the knight decides to move only rightward or downward in each step. Return the knight's minimum initial health so that he can rescue the princess. Note that any room can contain threats or power-ups, even the first room the knight enters and the bottom-right room where the princess is imprisoned.

Array,Dynamic Programming,Matrix

Hard

62,64,741,2354

Combine Two Tables

combine-two-tables

Table: Person Table: Address Write an SQL query to report the first name, last name, city, and state of each person in the Person table. If the address of a personId is not present in the Address table, report null instead. Return the result table in any order. The query result format is in the following example.

Database

Easy

577

Second Highest Salary

second-highest-salary

Table: Employee Write an SQL query to report the second highest salary from the Employee table. If there is no second highest salary, the query should report null. The query result format is in the following example.

Database

Medium

Nth Highest Salary

nth-highest-salary

Table: Employee Write an SQL query to report the nth highest salary from the Employee table. If there is no nth highest salary, the query should report null. The query result format is in the following example.

Database

Medium

2336

Rank Scores

rank-scores

Table: Scores Write an SQL query to rank the scores. The ranking should be calculated according to the following rules: Return the result table ordered by score in descending order. The query result format is in the following example.

Database

Medium

Largest Number

largest-number

Given a list of non-negative integers nums, arrange them such that they form the largest number and return it. Since the result may be very large, so you need to return a string instead of an integer.

String,Greedy,Sorting

Medium

2284

Consecutive Numbers

consecutive-numbers

Table: Logs Write an SQL query to find all numbers that appear at least three times consecutively. Return the result table in any order. The query result format is in the following example.

Database

Medium

Employees Earning More Than Their Managers

employees-earning-more-than-their-managers

Table: Employee Write an SQL query to find the employees who earn more than their managers. Return the result table in any order. The query result format is in the following example.

Database

Easy

Duplicate Emails

duplicate-emails

Table: Person Write an SQL query to report all the duplicate emails. Return the result table in any order. The query result format is in the following example.

Database

Easy

Customers Who Never Order

customers-who-never-order

Table: Customers Table: Orders Write an SQL query to report all customers who never order anything. Return the result table in any order. The query result format is in the following example.

Database

Easy

Department Highest Salary

department-highest-salary

Table: Employee Table: Department Write an SQL query to find employees who have the highest salary in each of the departments. Return the result table in any order. The query result format is in the following example.

Database

Medium

1214

Department Top Three Salaries

department-top-three-salaries

Table: Employee Table: Department A company's executives are interested in seeing who earns the most money in each of the company's departments. A high earner in a department is an employee who has a salary in the top three unique salaries for that department. Write an SQL query to find the employees who are high earners in each of the departments. Return the result table in any order. The query result format is in the following example.

Database

Hard

Reverse Words in a String II

reverse-words-in-a-string-ii

Two Pointers,String

Medium

151,189

Repeated DNA Sequences

repeated-dna-sequences

The DNA sequence is composed of a series of nucleotides abbreviated as 'A', 'C', 'G', and 'T'. When studying DNA, it is useful to identify repeated sequences within the DNA. Given a string s that represents a DNA sequence, return all the 10-letter-long sequences (substrings) that occur more than once in a DNA molecule. You may return the answer in any order.

Hash Table,String,Bit Manipulation,Sliding Window,Rolling Hash,Hash Function

Medium

Best Time to Buy and Sell Stock IV

best-time-to-buy-and-sell-stock-iv

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).

Array,Dynamic Programming

Hard

121,122,123

Rotate Array

rotate-array

Given an array, rotate the array to the right by k steps, where k is non-negative.

Array,Math,Two Pointers

Medium

The easiest solution would use additional memory and that is perfectly fine. The actual trick comes when trying to solve this problem without using any additional memory. This means you need to use the original array somehow to move the elements around. Now, we can place each element in its original location and shift all the elements around it to adjust as that would be too costly and most likely will time out on larger input arrays. One line of thought is based on reversing the array (or parts of it) to obtain the desired result. Think about how reversal might potentially help us out by using an example. The other line of thought is a tad bit complicated but essentially it builds on the idea of placing each element in its original position while keeping track of the element originally in that position. Basically, at every step, we place an element in its rightful position and keep track of the element already there or the one being overwritten in an additional variable. We can't do this in one linear pass and the idea here is based on cyclic-dependencies between elements.

61,186

Reverse Bits

reverse-bits

Reverse bits of a given 32 bits unsigned integer. Note:

Divide and Conquer,Bit Manipulation

Easy

7,191,2238

Number of 1 Bits

number-of-1-bits

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

Bit Manipulation

Easy

190,231,338,401,461,693,767

Word Frequency

word-frequency

Write a bash script to calculate the frequency of each word in a text file words.txt. For simplicity sake, you may assume: Example: Assume that words.txt has the following content: Your script should output the following, sorted by descending frequency: Note:

Shell

Medium

347

Valid Phone Numbers

valid-phone-numbers

Given a text file file.txt that contains a list of phone numbers (one per line), write a one-liner bash script to print all valid phone numbers. You may assume that a valid phone number must appear in one of the following two formats: (xxx) xxx-xxxx or xxx-xxx-xxxx. (x means a digit) You may also assume each line in the text file must not contain leading or trailing white spaces. Example: Assume that file.txt has the following content: Your script should output the following valid phone numbers:

Shell

Easy

Transpose File

transpose-file

Given a text file file.txt, transpose its content. You may assume that each row has the same number of columns, and each field is separated by the ' ' character. Example: If file.txt has the following content: Output the following:

Shell

Medium

Tenth Line

tenth-line

Given a text file file.txt, print just the 10th line of the file. Example: Assume that file.txt has the following content: Your script should output the tenth line, which is:

Shell

Easy

Delete Duplicate Emails

delete-duplicate-emails

Table: Person Write an SQL query to delete all the duplicate emails, keeping only one unique email with the smallest id. Note that you are supposed to write a DELETE statement and not a SELECT one. Return the result table in any order. The query result format is in the following example.

Database

Easy

Rising Temperature

rising-temperature

Table: Weather Write an SQL query to find all dates' Id with higher temperatures compared to its previous dates (yesterday). Return the result table in any order. The query result format is in the following example.

Database

Easy

House Robber

house-robber

You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed, the only constraint stopping you from robbing each of them is that adjacent houses have security systems connected and it will automatically contact the 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.

Array,Dynamic Programming

Medium

152,213,256,276,337,600,656,740,2262

Binary Tree Right Side View

binary-tree-right-side-view

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.

Tree,Depth-First Search,Breadth-First Search,Binary Tree

Medium

116,545

Number of Islands

number-of-islands

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. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Array,Depth-First Search,Breadth-First Search,Union Find,Matrix

Medium

130,286,305,323,694,695,2035,2103