Practice

📘 The Complete LeetCode Fundamentals Guide

Welcome to the comprehensive guide to mastering core computer science fundamentals and recognizing problem-solving patterns for technical interviews.

This repository contains a curated, categorized list of essential LeetCode problems, along with a structured 8-week study plan designed to build your skills progressively.

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🚀 How to Use This Guide

1. Follow the Study Plan: Start with Phase 1 and work your way down. Do not skip ahead until you are comfortable with the core concepts of your current phase.
2. Time Box Your Practice: Limit yourself to 20 minutes for Easy problems and 40 minutes for Mediums.
3. Understand, Don't Memorize: If you are stuck after the time limit, read the solution, understand the pattern, and try coding it again from scratch.
4. Use Your Interview Language: Implement all solutions in the language you intend to use in interviews (e.g., C#, Python, Java) to build strict muscle memory.

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🗓️ The 8-Week Study Plan

Phase 1: The Bedrock (Weeks 1-2)

Focus on how data is stored continuously in memory and how to manipulate indices efficiently.

• Concepts to Master: Time & Space Complexity (Big-O), Two Pointers, Sliding Window, Prefix Sums.
• Core Problem Set:
  • Arrays: Two Sum (#1), Remove Duplicates from Sorted Array (#26), Best Time to Buy and Sell Stock (#121).
  • Strings: Valid Palindrome (#125), Longest Substring Without Repeating Characters (#3).
  • Patterns: Container With Most Water (#11).

Phase 2: Memory & References (Weeks 3-4)

Transition from contiguous memory to node-based data structures.

• Concepts to Master: Singly vs. Doubly Linked Lists, LIFO (Stacks), FIFO (Queues), Hash Maps.
• Core Problem Set:
  • Linked Lists: Reverse Linked List (#206), Merge Two Sorted Lists (#21), Linked List Cycle (#141).
  • Stacks/Queues: Valid Parentheses (#20), Min Stack (#155).
  • Hashing: Group Anagrams (#49), Longest Consecutive Sequence (#128).

Phase 3: Non-Linear Hierarchies (Weeks 5-6)

Mastering recursive thinking and traversal algorithms.

• Concepts to Master: Binary Trees, Binary Search Trees (BST), Depth-First Search (DFS), Breadth-First Search (BFS), Recursion.
• Core Problem Set:
  • Trees: Binary Tree Level Order Traversal (#102), Maximum Depth of Binary Tree (Foundational).
  • BST: Validate Binary Search Tree (#98), Lowest Common Ancestor (#236).

Phase 4: Connections, State, & Data Retrieval (Weeks 7-8)

Tackling complex networks, pathfinding, and robust data querying.

• Concepts to Master: Graph representations, Dynamic Programming (Memoization vs. Tabulation), Relational Database logic (Joins, Aggregations).
• Core Problem Set:
  • Graphs: Number of Islands (#200), Clone Graph (#133), Course Schedule (#207).
  • DP: Climbing Stairs (#70), Coin Change (#322), Longest Increasing Subsequence (#300).
  • SQL: Combine Two Tables (#175), Second Highest Salary (#176).

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🗂️ The Complete Problem Bank by Data Structure

🧱 Arrays, Matrices & Dynamic Programming

ID	Problem Title	Difficulty
1	Two Sum	Easy
4	Median of Two Sorted Arrays	Hard
11	Container With Most Water	Medium
15	3Sum	Medium
16	3Sum Closest	Medium
18	4Sum	Medium
26	Remove Duplicates from Sorted Array	Easy
27	Remove Element	Easy
31	Next Permutation	Medium
32	Longest Valid Parentheses	Hard
33	Search in Rotated Sorted Array	Medium
34	Find First and Last Position of Element in Sorted Array	Medium
35	Search Insert Position	Easy
39	Combination Sum	Medium
40	Combination Sum II	Medium
41	First Missing Positive	Hard
42	Trapping Rain Water	Hard
45	Jump Game II	Medium
46	Permutations	Medium
47	Permutations II	Medium
48	Rotate Image	Medium
51	N-Queens	Hard
53	Maximum Subarray	Medium
54	Spiral Matrix	Medium
55	Jump Game	Medium
56	Merge Intervals	Medium
57	Insert Interval	Medium
60	Permutation Sequence	Hard
62	Unique Paths	Medium
70	Climbing Stairs	Easy
72	Edit Distance	Medium
73	Set Matrix Zeroes	Medium
74	Search a 2D Matrix	Medium
75	Sort Colors	Medium
78	Subsets	Medium
80	Remove Duplicates from Sorted Array II	Medium
84	Largest Rectangle in Histogram	Hard
85	Maximal Rectangle	Hard
88	Merge Sorted Array	Easy
90	Subsets II	Medium
115	Distinct Subsequences	Hard
118	Pascal's Triangle	Easy
119	Pascal's Triangle II	Easy
121	Best Time to Buy and Sell Stock	Easy
122	Best Time to Buy and Sell Stock II	Medium
123	Best Time to Buy and Sell Stock III	Hard
128	Longest Consecutive Sequence	Medium
131	Palindrome Partitioning	Medium
134	Gas Station	Medium
135	Candy	Hard
152	Maximum Product Subarray	Medium
153	Find Minimum in Rotated Sorted Array	Medium
162	Find Peak Element	Medium
167	Two Sum II - Input Array Is Sorted	Medium
169	Majority Element	Easy
179	Largest Number	Medium
189	Rotate Array	Medium
198	House Robber	Medium
209	Minimum Size Subarray Sum	Medium
213	House Robber II	Medium
215	Kth Largest Element in an Array	Medium
216	Combination Sum III	Medium
217	Contains Duplicate	Easy
219	Contains Duplicate II	Easy
220	Contains Duplicate III	Hard
221	Maximal Square	Medium
223	Rectangle Area	Medium
228	Summary Ranges	Easy
229	Majority Element II	Medium
238	Product of Array Except Self	Medium
239	Sliding Window Maximum	Hard
242	Valid Anagram	Easy
253	Meeting Rooms II	Medium
271	Encode and Decode Strings	Medium
278	First Bad Version	Easy
279	Perfect Squares	Medium
283	Move Zeroes	Easy
287	Find the Duplicate Number	Medium
300	Longest Increasing Subsequence	Medium
303	Range Sum Query - Immutable	Easy
304	Range Sum Query 2D - Immutable	Medium
312	Burst Balloons	Hard
315	Count of Smaller Numbers After Self	Hard
322	Coin Change	Medium
349	Intersection of Two Arrays	Easy
350	Intersection of Two Arrays II	Easy
354	Russian Doll Envelopes	Hard
360	Sort Transformed Array	Medium
407	Trapping Rain Water II	Hard
410	Split Array Largest Sum	Hard
413	Arithmetic Slices	Medium
414	Third Maximum Number	Easy
435	Non-overlapping Intervals	Medium
442	Find All Duplicates in an Array	Medium
448	Find All Numbers Disappeared in an Array	Easy
455	Assign Cookies	Easy
480	Sliding Window Median	Hard
485	Max Consecutive Ones	Easy
498	Diagonal Traverse	Medium
509	Fibonacci Number	Easy
518	Coin Change II	Medium
525	Contiguous Array	Medium
540	Single Element in a Sorted Array	Medium
560	Subarray Sum Equals K	Medium
605	Can Place Flowers	Easy
643	Maximum Average Subarray I	Easy
658	Find K Closest Elements	Medium
698	Partition to K Equal Sum Subsets	Medium
704	Binary Search	Easy
714	Best Time to Buy and Sell Stock with Transaction Fee	Medium
766	Toeplitz Matrix	Easy
799	Champagne Tower	Medium
840	Magic Squares In Grid	Medium
852	Peak Index in a Mountain Array	Medium
871	Minimum Number of Refueling Stops	Hard
875	Koko Eating Bananas	Medium
896	Monotonic Array	Easy
904	Fruit Into Baskets	Medium
905	Sort Array By Parity	Easy
907	Sum of Subarray Minimums	Medium
912	Sort an Array	Medium
914	X of a Kind in a Deck of Cards	Easy
922	Sort Array By Parity II	Easy
930	Binary Subarrays With Sum	Medium
939	Minimum Area Rectangle	Medium
956	Tallest Billboard	Hard
961	N-Repeated Element in Size 2N Array	Easy
974	Subarray Sums Divisible by K	Medium
977	Squares of a Sorted Array	Easy
978	Longest Turbulent Subarray	Medium
992	Subarrays with K Different Integers	Hard
1004	Max Consecutive Ones III	Medium
1011	Capacity To Ship Packages Within D Days	Medium
1027	Longest Arithmetic Subsequence	Medium
1095	Find in Mountain Array	Hard
1128	Number of Equivalent Domino Pairs	Easy
1200	Minimum Absolute Difference	Easy
1262	Greatest Sum Divisible by Three	Medium
1266	Minimum Time Visiting All Points	Easy
1277	Count Square Submatrices with All Ones	Medium
1283	Find the Smallest Divisor Given a Threshold	Medium
1292	Maximum Side Length of a Square with Sum Less than or Equal to Threshold	Medium
1296	Divide Array in Sets of K Consecutive Numbers	Medium
1351	Count Negative Numbers in a Sorted Matrix	Easy
1458	Max Dot Product of Two Subsequences	Hard
1470	Shuffle the Array	Easy
1480	Running Sum of 1d Array	Easy
1482	Minimum Number of Days to Make m Bouquets	Medium
1502	Can Make Arithmetic Progression From Sequence	Easy
1512	Number of Good Pairs	Easy
1539	Kth Missing Positive Number	Easy
1752	Check if Array Is Sorted and Rotated	Easy
1838	Frequency of the Most Frequent Element	Medium
1877	Minimize Maximum Pair Sum in Array	Medium
1895	Largest Magic Square	Medium
1925	Count Square Sum Triples	Easy
1929	Concatenation of Array	Easy
1975	Maximum Matrix Sum	Medium
1984	Minimum Difference Between Highest and Lowest of K Scores	Easy
2035	Partition Array Into Two Arrays to Minimize Sum Difference	Hard
2054	Two Best Non-Overlapping Events	Medium
2110	Number of Smooth Descent Periods of a Stock	Medium
2149	Rearrange Array Elements by Sign	Medium
2188	Minimum Time to Finish the Race	Hard
2402	Meeting Rooms III	Hard
2535	Difference Between Element Sum and Digit Sum of an Array	Easy
2654	Minimum Number of Operations to Make All Array Elements Equal to 1	Medium
2812	Find the Safest Path in a Grid	Medium
2943	Maximize Area of Square Hole in Grid	Medium
2965	Find Missing and Repeated Values	Easy
2974	Minimum Number Game	Easy
2975	Maximum Square Area by Removing Fences From a Field	Medium
3010	Divide an Array Into Subarrays With Minimum Cost I	Easy
3047	Find the Largest Area of Square Inside Two Rectangles	Medium
3314	Construct the Minimum Bitwise Array I	Easy
3379	Transformed Array	Easy
3380	Maximum Area Rectangle With Point Constraints I	Medium
3381	Maximum Subarray Sum With Length Divisible by K	Medium
3453	Separate Squares I	Medium
3454	Separate Squares II	Hard
3507	Minimum Pair Removal to Sort Array I	Easy
3512	Minimum Operations to Make Array Sum Divisible by K	Easy
3531	Count Covered Buildings	Medium
3562	Maximum Profit from Trading Stocks with Discounts	Hard
3573	Best Time to Buy and Sell Stock V	Medium
3577	Count the Number of Computer Unlocking Permutations	Medium
3583	Count Special Triplets	Medium
3623	Count Number of Trapezoids I	Medium
3625	Count Number of Trapezoids II	Hard
3634	Minimum Removals to Balance Array	Medium
3637	Trionic Array I	Easy
3640	Trionic Array II	Hard
3713	Longest Balanced Substring I	Medium
3714	Longest Balanced Substring II	Medium
3719	Longest Balanced Subarray I	Medium
3721	Longest Balanced Subarray II	Hard
3731	Find Missing Elements	Easy
3738	Longest Non-Decreasing Subarray After Replacing at Most One Element	Medium
3751	Total Waviness of Numbers in Range I	Medium
3756	Concatenate Non-Zero Digits and Multiply by Sum II	Medium

🧵 Strings

ID	Problem Title	Difficulty
3	Longest Substring Without Repeating Characters	Medium
5	Longest Palindromic Substring	Medium
6	Zigzag Conversion	Medium
10	Regular Expression Matching	Hard
14	Longest Common Prefix	Easy
17	Letter Combinations of a Phone Number	Medium
20	Valid Parentheses	Easy
22	Generate Parentheses	Medium
28	Find the Index of the First Occurrence in a String	Easy
38	Count and Say	Medium
43	Multiply Strings	Medium
44	Wildcard Matching	Hard
49	Group Anagrams	Medium
58	Length of Last Word	Easy
68	Text Justification	Hard
71	Simplify Path	Medium
76	Minimum Window Substring	Hard
97	Interleaving String	Medium
125	Valid Palindrome	Easy
139	Word Break	Medium
140	Word Break II	Hard
151	Reverse Words in a String	Medium
205	Isomorphic Strings	Easy
249	Group Shifted Strings	Medium
344	Reverse String	Easy
345	Reverse Vowels of a String	Easy
383	Ransom Note	Easy
387	First Unique Character in a String	Easy
392	Is Subsequence	Easy
394	Decode String	Medium
395	Longest Substring with At Least K Repeating Characters	Medium
409	Longest Palindrome	Easy
415	Add Strings	Easy
424	Longest Repeating Character Replacement	Medium
438	Find All Anagrams in a String	Medium
443	String Compression	Medium
520	Detect Capital	Easy
541	Reverse String II	Easy
567	Permutation in String	Medium
678	Valid Parenthesis String	Medium
692	Top K Frequent Words	Medium
696	Count Binary Substrings	Easy
767	Reorganize String	Medium
777	Swap Adjacent in LR String	Medium
796	Rotate String	Easy
833	Find And Replace in String	Medium
844	Backspace String Compare	Easy
1021	Remove Outermost Parentheses	Easy
1044	Longest Duplicate Substring	Hard
1047	Remove All Adjacent Duplicates In String	Easy
1071	Greatest Common Divisor of Strings	Easy
1143	Longest Common Subsequence	Medium
1358	Number of Substrings Containing All Three Characters	Medium
1528	Shuffle String	Easy
1653	Minimum Deletions to Make String Balanced	Medium
1657	Determine if Two Strings Are Close	Medium
1768	Merge Strings Alternately	Easy
1859	Sorting the Sentence	Easy
2337	Move Pieces to Obtain a String	Medium
2423	Remove Letter To Equalize Frequency	Easy
2976	Minimum Cost to Convert String I	Medium

🔗 Linked Lists

ID	Problem Title	Difficulty
2	Add Two Numbers	Medium
19	Remove Nth Node From End of List	Medium
21	Merge Two Sorted Lists	Easy
23	Merge k Sorted Lists	Hard
24	Swap Nodes in Pairs	Medium
25	Reverse Nodes in k-Group	Hard
83	Remove Duplicates from Sorted List	Easy
86	Partition List	Medium
92	Reverse Linked List II	Medium
138	Copy List with Random Pointer	Medium
141	Linked List Cycle	Easy
142	Linked List Cycle II	Medium
143	Reorder List	Medium
148	Sort List	Medium
160	Intersection of Two Linked Lists	Easy
206	Reverse Linked List	Easy
234	Palindrome Linked List	Easy
237	Delete Node in a Linked List	Medium
430	Flatten a Multilevel Doubly Linked List	Medium
876	Middle of the Linked List	Easy

🌳 Trees & Tries

ID	Problem Title	Difficulty
94	Binary Tree Inorder Traversal	Easy
95	Unique Binary Search Trees II	Medium
98	Validate Binary Search Tree	Medium
99	Recover Binary Search Tree	Medium
100	Same Tree	Easy
102	Binary Tree Level Order Traversal	Medium
103	Binary Tree Zigzag Level Order Traversal	Medium
108	Convert Sorted Array to Binary Search Tree	Easy
110	Balanced Binary Tree	Easy
112	Path Sum	Easy
113	Path Sum II	Medium
116	Populating Next Right Pointers in Each Node	Medium
124	Binary Tree Maximum Path Sum	Hard
230	Kth Smallest Element in a BST	Medium
236	Lowest Common Ancestor of a Binary Tree	Medium
297	Serialize and Deserialize Binary Tree	Hard
337	House Robber III	Medium
437	Path Sum III	Medium
450	Delete Node in a BST	Medium
543	Diameter of Binary Tree	Easy
572	Subtree of Another Tree	Easy
653	Two Sum IV - Input is a BST	Easy
669	Trim a Binary Search Tree	Medium
701	Insert into a Binary Search Tree	Medium
865	Smallest Subtree with all the Deepest Nodes	Medium
987	Vertical Order Traversal of a Binary Tree	Hard
1008	Construct Binary Search Tree from Preorder Traversal	Medium
1161	Maximum Level Sum of a Binary Tree	Medium
1339	Maximum Product of Splitted Binary Tree	Medium
1373	Maximum Sum BST in Binary Tree	Hard
1483	Kth Ancestor of a Tree Node	Hard

🌐 Graphs & Search (BFS/DFS)

ID	Problem Title	Difficulty
37	Sudoku Solver	Hard
79	Word Search	Medium
126	Word Ladder II	Hard
127	Word Ladder	Hard
130	Surrounded Regions	Medium
133	Clone Graph	Medium
200	Number of Islands	Medium
207	Course Schedule	Medium
210	Course Schedule II	Medium
399	Evaluate Division	Medium
417	Pacific Atlantic Water Flow	Medium
695	Max Area of Island	Medium
721	Accounts Merge	Medium
733	Flood Fill	Easy
743	Network Delay Time	Medium
787	Cheapest Flights Within K Stops	Medium
802	Find Eventual Safe States	Medium
815	Bus Routes	Hard
827	Making A Large Island	Hard
843	Guess the Word	Hard
909	Snakes and Ladders	Medium
934	Shortest Bridge	Medium
947	Most Stones Removed with Same Row or Column	Medium
980	Unique Paths III	Hard
994	Rotting Oranges	Medium
1631	Path With Minimum Effort	Medium
1971	Find if Path Exists in Graph	Easy
1976	Number of Ways to Arrive at Destination	Medium

🧮 Stacks, Queues, Heaps & System Design

ID	Problem Title	Difficulty
146	LRU Cache	Medium
150	Evaluate Reverse Polish Notation	Medium
155	Min Stack	Medium
224	Basic Calculator	Hard
227	Basic Calculator II	Medium
232	Implement Queue using Stacks	Easy
295	Find Median from Data Stream	Hard
347	Top K Frequent Elements	Medium
359	Logger Rate Limiter	Easy
362	Design Hit Counter	Medium
380	Insert Delete GetRandom O(1)	Medium
402	Remove K Digits	Medium
496	Next Greater Element I	Easy
503	Next Greater Element II	Medium
622	Design Circular Queue	Medium
636	Exclusive Time of Functions	Medium
703	Kth Largest Element in a Stream	Easy
715	Range Module	Hard
735	Asteroid Collision	Medium
900	RLE Iterator	Medium
981	Time Based Key-Value Store	Medium
1244	Design A Leaderboard	Medium
1381	Design a Stack With Increment Operation	Medium
1603	Design Parking System	Easy
2296	Design a Text Editor	Hard
3815	Design Auction System	Medium

🧮 Mathematics & Bit Manipulation

ID	Problem Title	Difficulty
7	Reverse Integer	Medium
9	Palindrome Number	Easy
12	Integer to Roman	Medium
13	Roman to Integer	Easy
29	Divide Two Integers	Medium
50	Pow(x, n)	Medium
66	Plus One	Easy
67	Add Binary	Easy
69	Sqrt(x)	Easy
136	Single Number	Easy
137	Single Number II	Medium
168	Excel Sheet Column Title	Easy
190	Reverse Bits	Easy
202	Happy Number	Easy
231	Power of Two	Easy
233	Number of Digit One	Hard
258	Add Digits	Easy
263	Ugly Number	Easy
268	Missing Number	Easy
282	Expression Add Operators	Hard
292	Nim Game	Easy
371	Sum of Two Integers	Medium
401	Binary Watch	Easy
507	Perfect Number	Easy
645	Set Mismatch	Easy
693	Binary Number with Alternating Bits	Easy
728	Self Dividing Numbers	Easy
1015	Smallest Integer Divisible by K	Medium
1025	Divisor Game	Easy
1137	N-th Tribonacci Number	Easy
1523	Count Odd Numbers in an Interval Range	Easy
1922	Count Good Numbers	Medium
2235	Add Two Integers	Easy
2396	Strictly Palindromic Number	Medium

🗄️ Databases (SQL) & JavaScript Fundamentals

ID	Problem Title	Difficulty
175	Combine Two Tables	Easy
176	Second Highest Salary	Medium
177	Nth Highest Salary	Medium
178	Rank Scores	Medium
181	Employees Earning More Than Their Managers	Easy
183	Customers Who Never Order	Easy
196	Delete Duplicate Emails	Easy
197	Rising Temperature	Easy
262	Trips and Users	Hard
550	Game Play Analysis IV	Medium
577	Employee Bonus	Easy
585	Investments in 2016	Medium
586	Customer Placing the Largest Number of Orders	Easy
595	Big Countries	Easy
601	Human Traffic of Stadium	Hard
620	Not Boring Movies	Easy
1068	Product Sales Analysis I	Easy
1141	User Activity for the Past 30 Days I	Easy
1148	Article Views I	Easy
1193	Monthly Transactions I	Medium
1251	Average Selling Price	Easy
1378	Replace Employee ID With The Unique Identifier	Easy
1661	Average Time of Process per Machine	Easy
1683	Invalid Tweets	Easy
1757	Recyclable and Low Fat Products	Easy
1934	Confirmation Rate	Medium
2619	Array Prototype Last	Easy
2620	Counter	Easy
2667	Create Hello World Function	Easy
2704	To Be Or Not To Be	Easy