15.2 Lookup by Difficulty

15.2.1 Easy Problems

Platform	ID	Problem	Chapter
LeetCode	1	Two Sums I	Array
LeetCode	14	Longest Common Prefix	String Manipulation
LeetCode	21	Merge Two Sorted Lists	Linked List
LeetCode	26	Remove Duplicates from Sorted Array	Array
LeetCode	53	Maximum Subarray	Array
LeetCode	66	Plus One	Array
LeetCode	70	Climbing Stairs	Dynamic Programming and Backtracking
LeetCode	83	Remove Duplicates from Sorted List	Linked List
LeetCode	94	Binary Tree Inorder Traversal	Tree
LeetCode	110	Balanced Binary Tree	Tree
LeetCode	111	Minimum Depth of Binary Tree	Tree
LeetCode	112	Path Sum	Tree
LeetCode	118	Pascal’s Triangle	Dynamic Programming and Backtracking
LeetCode	121	Best Time to Buy and Sell Stock	Array
LeetCode	122	Best Time to Buy and Sell Stock II	Array
LeetCode	136	Single Number	Math
LeetCode	141	Linked List Cycle	Linked List
LeetCode	144	Binary Tree Preorder Traversal	Tree
LeetCode	145	Binary Tree Postorder Traversal	Tree
LeetCode	160	Intersection of Two Linked Lists	Linked List
LeetCode	190	Reverse Bits	Math
LeetCode	191	Number of 1 Bits	Math
LeetCode	198	House Robber	Dynamic Programming and Backtracking
LeetCode	202	Happy Number	Math
LeetCode	204	Count Primes	Math
LeetCode	206	Reverse Linked List	Linked List
LeetCode	232	Implement Queue using Stacks	Stack and Queue
LeetCode	234	Palindrome Linked List	Linked List
LeetCode	256	Paint House	Dynamic Programming and Backtracking
LeetCode	278	First Bad Version	Interactive
LeetCode	383	Ransom Note	String Manipulation
LeetCode	409	Longest Palindrome	String Manipulation
LeetCode	543	Diameter of Binary Tree	Tree
LeetCode	572	Subtree of Another Tree	Tree
LeetCode	653	Two Sum IV - BST	Tree
LeetCode	680	Valid Palindrome II	String Manipulation
LeetCode	704	Binary Search	Array
LeetCode	705	Design HashSet	Advanced Data Structure
LeetCode	706	Design HashMap	Advanced Data Structure
LeetCode	905	Sort Array By Parity	Array
LeetCode	953	Verifying an Alien Dictionary	String Manipulation
LeetCode	1002	Find Common Characters	Array
LeetCode	1046	Last Stone Weight	Heap
LeetCode	-	Shortest Word Distance	Array
HackerRank	-	Height of Binary Search Tree	Tree
HackerRank	-	Validate Properly Nested Brackets	Stack and Queue
HackerRank	-	Count Elements Greater Than Previous Average	Array
HackerRank	-	Count Number Pairs	Array
HackerRank	-	Check Palindrome by Filtering Non-Letters	String Manipulation
HackerRank	-	Generate Valid Angle Bracket Sequences	Dynamic Programming and Backtracking
HackerRank	-	Custom Fibonacci Sequence	Dynamic Programming and Backtracking
Firecode	-	Max Gain	Array
Firecode	-	Selection Sort Array	Array
Firecode	-	Binary Representation	String Manipulation
Firecode	-	Insert Stars	String Manipulation
Firecode	-	Power of Two	Math
Firecode	-	Fibonacci Sequence	Dynamic Programming and Backtracking
Other	-	Two Sum II - Boolean	Array
Other	-	Retrieve Elements K Times	Array
Other	-	Insert Node at Position in Doubly Linked List	Linked List
Interview	-	Lock-Free Stack Using Linked List (CAS)	Stack and Queue
Interview	-	First Non-Repeated Character	String Manipulation
Interview	-	Implement Stack Using Queues	Advanced Data Structure
Interview	-	Implement Queue	Advanced Data Structure

15.2.2 Medium Problems

Platform	ID	Problem	Chapter
LeetCode	2	Add Two Numbers	Linked List
LeetCode	3	Longest Substring Without Repeating	String Manipulation
LeetCode	5	Longest Palindromic Substring	Dynamic Programming and Backtracking
LeetCode	7	Reverse Integer	Math
LeetCode	8	String to Integer (atoi)	String Manipulation
LeetCode	11	Container With Most Water	Array
LeetCode	15	3Sum	Array
LeetCode	16	3Sum Closest	Array
LeetCode	17	Letter Combinations of a Phone Number	Dynamic Programming and Backtracking
LeetCode	18	4Sum	Array
LeetCode	19	Remove Nth Node From End	Linked List
LeetCode	33	Search in Rotated Sorted Array	Array
LeetCode	36	Valid Sudoku	Matrix
LeetCode	39	Combination Sum	Dynamic Programming and Backtracking
LeetCode	46	Permutations	Dynamic Programming and Backtracking
LeetCode	47	Permutations II	Dynamic Programming and Backtracking
LeetCode	48	Rotate Image	Matrix
LeetCode	49	Group Anagrams	String Manipulation
LeetCode	50	Pow(x; n)	Math
LeetCode	54	Spiral Matrix	Matrix
LeetCode	56	Merge Intervals	Array
LeetCode	62	Unique Paths	Dynamic Programming and Backtracking
LeetCode	64	Minimum Path Sum	Dynamic Programming and Backtracking
LeetCode	73	Set Matrix Zeroes	Matrix
LeetCode	74	Search a 2D Matrix	Matrix
LeetCode	75	Sort Colors	Array
LeetCode	78	Subsets	Dynamic Programming and Backtracking
LeetCode	79	Word Search	Dynamic Programming and Backtracking
LeetCode	90	Subsets II	Dynamic Programming and Backtracking
LeetCode	91	Decode Ways	Dynamic Programming and Backtracking
LeetCode	92	Reverse Linked List II	Linked List
LeetCode	98	Validate Binary Search Tree	Tree
LeetCode	102	Binary Tree Level Order Traversal	Tree
LeetCode	107	Binary Tree Level Order Traversal II	Tree
LeetCode	116	Populating Next Right Pointers in Each Node	Tree
LeetCode	127	Word Ladder	String Manipulation
LeetCode	130	Surrounded Regions	Matrix
LeetCode	133	Clone Graph	Graph
LeetCode	134	Gas Station	Array
LeetCode	138	Copy List with Random Pointer	Linked List
LeetCode	139	Word Break	Dynamic Programming and Backtracking
LeetCode	142	Linked List Cycle II	Linked List
LeetCode	143	Reorder List	Linked List
LeetCode	146	Design LRU Cache	Advanced Data Structure
LeetCode	147	Insertion Sort List	Linked List
LeetCode	148	Sort List	Linked List
LeetCode	150	Evaluate Reverse Polish Notation	Stack and Queue
LeetCode	152	Maximum Product Subarray	Array
LeetCode	155	Min Stack	Stack and Queue
LeetCode	156	Binary Tree Upside Down	Tree
LeetCode	167	Two Sum II - Sorted Array	Array
LeetCode	200	Number of Islands	Matrix
LeetCode	207	Course Schedule	Graph
LeetCode	208	Implement Trie (Prefix Tree)	Tree
LeetCode	209	Minimum Size Subarray Sum	Array
LeetCode	210	Course Schedule II	Graph
LeetCode	211	Design Add and Search Words Data Structure	String Manipulation
LeetCode	215	Find kth Largest Element	Heap
LeetCode	221	Maximal Square	Dynamic Programming and Backtracking
LeetCode	230	Kth Smallest Element in a BST	Tree
LeetCode	235	Lowest Common Ancestor of a Binary Search Tree	Tree
LeetCode	236	Lowest Common Ancestor of a Binary Tree	Tree
LeetCode	238	Product of Array Except Self	Array
LeetCode	240	Search a 2D Matrix II	Matrix
LeetCode	250	Count Univalue Subtrees	Tree
LeetCode	253	Meeting Rooms II	Heap
LeetCode	261	Graph Valid Tree	Graph
LeetCode	271	Encode and Decode Strings	String Manipulation
LeetCode	285	Inorder Successor in BST	Tree
LeetCode	286	Walls and Gates	Matrix
LeetCode	287	Find the Duplicate Number	Array
LeetCode	298	Binary Tree Longest Consecutive Sequence	Tree
LeetCode	300	Longest Increasing Subsequence	Dynamic Programming and Backtracking
LeetCode	309	Best Time to Buy and Sell Stock with Cooldown	Dynamic Programming and Backtracking
LeetCode	322	Coin Change	Dynamic Programming and Backtracking
LeetCode	323	Number of Connected Components in an Undirected Graph	Graph
LeetCode	328	Odd Even Linked List	Linked List
LeetCode	347	Top K Frequent Elements	Heap
LeetCode	348	Design Tic-Tac-Toe	Matrix
LeetCode	355	Design Twitter	Advanced Data Structure
LeetCode	366	Find Leaves of Binary Tree	Tree
LeetCode	371	Sum of Two Integers	Math
LeetCode	398	Random Pick Index	Array
LeetCode	417	Pacific Atlantic Water Flow	Matrix
LeetCode	419	Battleships in a Board	Matrix
LeetCode	424	Longest Repeating Character Replacement	String Manipulation
LeetCode	435	Non-overlapping Intervals	Array
LeetCode	438	Find All Anagrams in a String	String Manipulation
LeetCode	451	Sort Characters By Frequency	String Manipulation
LeetCode	454	4Sum II	Array
LeetCode	470	Implement Rand10() Using Rand7()	Math
LeetCode	518	Coin Change II	Dynamic Programming and Backtracking
LeetCode	528	Random Pick with Weight	Array
LeetCode	542	01 Matrix	Matrix
LeetCode	554	Brick Wall	Matrix
LeetCode	567	Permutation in String	String Manipulation
LeetCode	621	Task Scheduler	Stack and Queue
LeetCode	647	Palindromic Substrings	Dynamic Programming and Backtracking
LeetCode	721	Accounts Merge	Graph
LeetCode	729	My Calendar I	Advanced Data Structure
LeetCode	739	Daily Temperatures	Stack and Queue
LeetCode	743	Network Delay Time	Graph
LeetCode	801	Minimum Swaps Sequences Increasing	Dynamic Programming and Backtracking
LeetCode	846	Hand of Straights	Array
LeetCode	849	Maximize Distance to Closest Person	Array
LeetCode	853	Car Fleet	Stack and Queue
LeetCode	875	Koko Eating Bananas	Array
LeetCode	973	K Closest Points to Origin	Heap
LeetCode	981	Time Based Key-Value Store	Advanced Data Structure
LeetCode	986	Interval List Intersections	Array
LeetCode	994	Rotting Oranges	Matrix
LeetCode	1277	Count Square Submatrices with All Ones	Dynamic Programming and Backtracking
LeetCode	1347	Minimum Number of Steps to Make Two Strings Anagram	String Manipulation
LeetCode	1358	Number of Substrings All Three Chars	String Manipulation
LeetCode	1448	Count Good Nodes in Binary Tree	Tree
LeetCode	1669	Merge In Between Linked Lists	Linked List
LeetCode	1717	Maximum Score From Removing Substrings	Stack and Queue
LeetCode	1891	Cutting Ribbons	Array
LeetCode	1899	Merge Triplets to Form Target Triplet	Array
LeetCode	1921	Eliminate Maximum Monsters	Array
LeetCode	2013	Detect Squares	Matrix
LeetCode	2043	Simple Bank System	Advanced Data Structure
LeetCode	2348	Number of Zero-Filled Subarrays	Array
LeetCode	2966	Divide Array Into Arrays With Max Difference	Array
LeetCode	3191	Minimum Operations to Make Binary Array Elements Equal to One I	Array
HackerRank	-	Next Greater Element with Position Offset	Stack and Queue
HackerRank	-	Find Index Combinations with Target Weight Sum	Dynamic Programming and Backtracking
HackerRank	-	Merge and Sort Intervals	Array
HackerRank	-	Minimum Plans to Reach Target Bandwidth	Dynamic Programming and Backtracking
HackerRank	-	Top K Frequent Events with Order Preservation	Heap
HackerRank	-	Max Unique Substring Length in a Session	String Manipulation
HackerRank	-	One-Pass Removal of k-th Node from End	Linked List
HackerRank	-	Reverse Even-Indexed Nodes and Append	Linked List
Firecode	-	Maximum Number of Repetitions	Array
Firecode	-	Retrieve Optimal Computation	Dynamic Programming and Backtracking
Other	-	Two Sum V	Array
Other	-	Distance of Nearest Cell Having 1	Graph
Interview	-	Hardware Build Feasibility	Graph
Interview	-	Merge K Sorted Arrays	Heap
Interview	-	Quick Sort Array	Array
Interview	-	Top K Elements	Heap
Interview	-	Longest Common Substring	Dynamic Programming and Backtracking
Interview	-	Minimum Operations to One	Dynamic Programming and Backtracking
Interview	-	VIP Customer Scheduler	Heap

15.2.3 Hard Problems

Platform	ID	Problem	Chapter
LeetCode	4	Median of Two Sorted Arrays	Array
LeetCode	23	Merge k Sorted Lists	Linked List
LeetCode	25	Reverse Nodes in k-Group	Linked List
LeetCode	32	Longest Valid Parentheses	Stack and Queue
LeetCode	57	Insert Interval	Array
LeetCode	269	Alien Dictionary	Graph
LeetCode	295	Find Median from Data Stream	Heap
LeetCode	296	Best Meeting Point	Matrix
LeetCode	297	Serialize and Deserialize Binary Tree	Tree
LeetCode	843	Guess the Word	Interactive

15.2.4 Unclassified Problems

Platform	ID	Problem	Chapter
Other	-	Graph Serialization	Graph

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