130 Most Popular LeetCode Problems and Answers

Array:

1. Two Sum
2. Container With Most Water
3. Best Time to Buy and Sell Stock
4. Product of Array Except Self

String:

1. Longest Substring Without Repeating Characters
2. Valid Parentheses
3. Valid Palindrome
4. Reverse Words in a String
5. Longest Palindromic Substring

Linked List:

1. Reverse Linked List
2. Merge Two Sorted Lists
3. Remove Nth Node From End of List
4. Linked List Cycle
5. Intersection of Two Linked Lists

Stack:

1. Min Stack
2. Evaluate Reverse Polish Notation
3. Daily Temperatures
4. Remove All Adjacent Duplicates in String

Queue:

1. Design Circular Queue
2. Implement Stack using Queues
3. Moving Average from Data Stream
4. Queue Reconstruction by Height
5. Number of Recent Calls

Binary Tree:

1. Maximum Depth of Binary Tree
2. Validate Binary Search Tree
3. Binary Tree Level Order Traversal
4. Lowest Common Ancestor of a Binary Tree
5. Symmetric Tree

Binary Search Tree (BST):

1. Kth Smallest Element in a BST
2. Convert Sorted Array to Binary Search Tree
3. Validate Binary Search Tree
4. Inorder Successor in BST
5. Range Sum of BST

Graph:

1. Number of Islands
2. Course Schedule
3. Word Search
4. Clone Graph
5. Pacific Atlantic Water Flow

Hashing:

1. Group Anagrams
2. Valid Sudoku
3. Subarray Sum Equals K
4. Longest Consecutive Sequence

Dynamic Programming:

1. Climbing Stairs
2. Longest Increasing Subsequence
3. Maximum Subarray
4. Coin Change
5. Unique Paths

Greedy Algorithms:

1. Jump Game
2. Task Scheduler
3. Minimum Number of Arrows to Burst Balloons
4. Non-overlapping Intervals
5. Gas Station

Backtracking:

1. N-Queens
2. Letter Combinations of a Phone Number
3. Permutations
4. Subsets
5. Palindrome Partitioning

Divide and Conquer:

1. Merge Sort
2. Quick Sort
3. Kth Largest Element in an Array
4. Maximum Subarray
5. Count of Smaller Numbers After Self

Two Pointers:

1. Two Sum II — Input array is sorted
2. Trapping Rain Water

Sliding Window:

1. Minimum Window Substring
2. Maximum Points You Can Obtain from Cards
3. Fruit Into Baskets
4. Longest Repeating Character Replacement

Bit Manipulation:

1. Single Number
2. Number of 1 Bits
3. Missing Number
4. Reverse Bits
5. Power of Two

Sorting and Searching:

1. Merge Intervals
2. First Missing Positive
3. Search in Rotated Sorted Array
4. Find First and Last Position of Element in Sorted Array

Math:

1. Palindrome Number
2. Roman to Integer
3. Integer to English Words
4. Pow(x, n)
5. Happy Number

Recursion:

1. Generate Parentheses
2. Combinations

Design and Implementation:

1. LRU Cache
2. Implement Trie (Prefix Tree)
3. Design Circular Queue

Union Find (Disjoint Set):

1. Friend Circles
2. Accounts Merge
3. Surrounded Regions
4. Redundant Connection

Trie:

1. Add and Search Word — Data structure design
2. Word Search II
3. Replace Words
4. Implement Magic Dictionary

Heap (Priority Queue):

1. Merge k Sorted Lists
2. Top K Frequent Elements
3. Find Median from Data Stream
4. Ugly Number II

Topological Sort:

1. Course Schedule
2. Alien Dictionary
3. Sequence Reconstruction
4. Reconstruct Itinerary
5. Course Schedule II

Breadth-First Search (BFS):

1. Word Ladder
2. Binary Tree Level Order Traversal
3. Minimum Genetic Mutation

Depth-First Search (DFS):

1. Binary Tree Paths
2. Flatten Binary Tree to Linked List
3. Word Break
4. Remove Invalid Parentheses
5. All Paths From Source to Target

Segment Tree:

1. Range Sum Query — Mutable
2. Count of Range Sum
3. The Skyline Problem
4. Range Minimum Query (RMQ)

Fenwick Tree (Binary Indexed Tree):

1. Reverse Pairs
2. Range Sum Query 2D — Mutable

Linked List Manipulation:

1. Swap Nodes in Pairs

Tree Traversals:

1. Binary Tree Preorder Traversal
2. Binary Tree Inorder Traversal
3. Binary Tree Postorder Traversal
4. N-ary Tree Preorder Traversal
5. N-ary Tree Postorder Traversal
6. Lowest Common Ancestor of a Binary Search Tree

Geometry:

1. Valid Square
2. K Closest Points to Origin
3. Number of Boomerangs
4. Convex Hull
5. Max Points on a Line