Top 50 LeetCode Interview Questions Every Developer Must Know

Master the Most Important LeetCode Questions for 2025

LeetCode has become the gold standard for coding interview preparation. With over 3,000 problems available, it can be overwhelming to know where to start. This comprehensive guide covers the 50 most important LeetCode questions that appear in interviews at top tech companies like Google, Meta, Amazon, Microsoft, and Apple.

Why These 50 Questions Matter

These questions were carefully selected based on:

• Frequency of appearance in real interviews
• Coverage of essential algorithmic patterns
• Difficulty progression from easy to hard
• Foundation for understanding more complex problems
• Time-tested relevance across multiple companies

How to Use This Guide

Each question includes the problem statement, optimal approach, time/space complexity analysis, and key insights. Practice these systematically, and you'll be prepared for 80% of coding interviews.

Array & String Problems (15 Questions)

1. Two Sum (LeetCode #1) - Easy

Problem: Given an array of integers and a target sum, return indices of two numbers that add up to the target.

Key Insight: Use a hash map to store complements for O(n) time complexity.

Pattern: Hash Map Lookup

2. Best Time to Buy and Sell Stock (LeetCode #121) - Easy

Problem: Find the maximum profit from buying and selling a stock once.

Key Insight: Track minimum price seen so far and maximum profit.

Pattern: Single Pass Tracking

3. Contains Duplicate (LeetCode #217) - Easy

Problem: Determine if any value appears twice in an array.

Key Insight: Use a set to track seen elements.

Pattern: Set-based Detection

4. Maximum Subarray (LeetCode #53) - Medium

Problem: Find the contiguous subarray with the largest sum.

Key Insight: Kadane's algorithm - reset sum when it becomes negative.

Pattern: Dynamic Programming

5. Product of Array Except Self (LeetCode #238) - Medium

Problem: Return array where each element is product of all other elements.

Key Insight: Use left and right pass to avoid division.

Pattern: Prefix/Suffix Arrays

6. 3Sum (LeetCode #15) - Medium

Problem: Find all unique triplets that sum to zero.

Key Insight: Sort array, use two pointers for each fixed element.

Pattern: Two Pointers

7. Container With Most Water (LeetCode #11) - Medium

Problem: Find two lines that form container holding most water.

Key Insight: Two pointers, move the shorter line inward.

Pattern: Two Pointers

8. Longest Substring Without Repeating Characters (LeetCode #3) - Medium

Problem: Find length of longest substring without duplicate characters.

Key Insight: Sliding window with hash set.

Pattern: Sliding Window

9. Group Anagrams (LeetCode #49) - Medium

Problem: Group strings that are anagrams of each other.

Key Insight: Use sorted string as key in hash map.

Pattern: Hash Map Grouping

10. Valid Parentheses (LeetCode #20) - Easy

Problem: Determine if string of parentheses is valid.

Key Insight: Use stack to match opening/closing brackets.

Pattern: Stack

Tree & Graph Problems (12 Questions)

11. Maximum Depth of Binary Tree (LeetCode #104) - Easy

Problem: Find the maximum depth of a binary tree.

Key Insight: Recursive approach - 1 + max of left/right subtrees.

Pattern: Tree Traversal

12. Same Tree (LeetCode #100) - Easy

Problem: Check if two binary trees are identical.

Key Insight: Recursive comparison of values and structure.

Pattern: Tree Comparison

13. Invert Binary Tree (LeetCode #226) - Easy

Problem: Invert a binary tree (mirror image).

Key Insight: Swap left and right children recursively.

Pattern: Tree Modification

14. Binary Tree Level Order Traversal (LeetCode #102) - Medium

Problem: Return level order traversal of binary tree.

Key Insight: Use BFS with queue.

Pattern: BFS

15. Validate Binary Search Tree (LeetCode #98) - Medium

Problem: Determine if binary tree is valid BST.

Key Insight: In-order traversal should be sorted, or use bounds.

Pattern: BST Properties

16. Lowest Common Ancestor of BST (LeetCode #235) - Medium

Problem: Find LCA of two nodes in BST.

Key Insight: Use BST property to navigate toward LCA.

Pattern: BST Navigation

17. Number of Islands (LeetCode #200) - Medium

Problem: Count number of islands in 2D grid.

Key Insight: DFS/BFS to mark connected land cells.

Pattern: Graph Traversal

18. Clone Graph (LeetCode #133) - Medium

Problem: Deep clone an undirected graph.

Key Insight: DFS with hash map to track cloned nodes.

Pattern: Graph Cloning

19. Course Schedule (LeetCode #207) - Medium

Problem: Determine if you can finish all courses (detect cycle).

Key Insight: Topological sort or DFS cycle detection.

Pattern: Topological Sort

20. Binary Tree Maximum Path Sum (LeetCode #124) - Hard

Problem: Find maximum path sum in binary tree.

Key Insight: For each node, consider path through it vs. extending upward.

Pattern: Tree DP

Dynamic Programming Problems (8 Questions)

21. Climbing Stairs (LeetCode #70) - Easy

Problem: Count ways to climb n stairs (1 or 2 steps at a time).

Key Insight: Fibonacci sequence - f(n) = f(n-1) + f(n-2).

Pattern: 1D DP

22. House Robber (LeetCode #198) - Medium

Problem: Rob houses to maximize money without robbing adjacent ones.

Key Insight: For each house, choose max of (rob + dp[i-2]) vs. dp[i-1].

Pattern: 1D DP

23. Coin Change (LeetCode #322) - Medium

Problem: Find minimum coins to make target amount.

Key Insight: DP - for each amount, try all coins and take minimum.

Pattern: Unbounded Knapsack

24. Longest Increasing Subsequence (LeetCode #300) - Medium

Problem: Find length of longest increasing subsequence.

Key Insight: DP with binary search optimization or simple O(n²) DP.

Pattern: LIS DP

25. Unique Paths (LeetCode #62) - Medium

Problem: Count unique paths in m×n grid (only right/down moves).

Key Insight: DP - paths[i][j] = paths[i-1][j] + paths[i][j-1].

Pattern: 2D DP

26. Jump Game (LeetCode #55) - Medium

Problem: Determine if you can reach last index.

Key Insight: Greedy - track farthest reachable position.

Pattern: Greedy/DP

27. Decode Ways (LeetCode #91) - Medium

Problem: Count ways to decode a string of digits.

Key Insight: DP - consider single digit and two-digit decodings.

Pattern: String DP

28. Word Break (LeetCode #139) - Medium

Problem: Check if string can be segmented into dictionary words.

Key Insight: DP - for each position, check all possible word endings.

Pattern: String DP

Linked List Problems (6 Questions)

29. Reverse Linked List (LeetCode #206) - Easy

Problem: Reverse a singly linked list.

Key Insight: Iterative with three pointers: prev, curr, next.

Pattern: Linked List Manipulation

30. Merge Two Sorted Lists (LeetCode #21) - Easy

Problem: Merge two sorted linked lists.

Key Insight: Two pointers, compare and advance smaller value.

Pattern: Merge Technique

31. Linked List Cycle (LeetCode #141) - Easy

Problem: Detect if linked list has a cycle.

Key Insight: Floyd's cycle detection (slow/fast pointers).

Pattern: Two Pointers

32. Remove Nth Node From End (LeetCode #19) - Medium

Problem: Remove nth node from end of linked list.

Key Insight: Two pointers with n gap between them.

Pattern: Two Pointers

33. Reorder List (LeetCode #143) - Medium

Problem: Reorder list: L0 → Ln → L1 → Ln-1 → ...

Key Insight: Find middle, reverse second half, merge alternately.

Pattern: Linked List Manipulation

34. Merge k Sorted Lists (LeetCode #23) - Hard

Problem: Merge k sorted linked lists.

Key Insight: Divide and conquer or priority queue.

Pattern: Merge Technique

Advanced Problems (9 Questions)

35. Find Median from Data Stream (LeetCode #295) - Hard

Problem: Maintain median in stream of numbers.

Key Insight: Two heaps - max heap for smaller half, min heap for larger half.

Pattern: Two Heaps

36. Serialize and Deserialize Binary Tree (LeetCode #297) - Hard

Problem: Convert binary tree to string and back.

Key Insight: Pre-order traversal with null markers.

Pattern: Tree Serialization

37. Trapping Rain Water (LeetCode #42) - Hard

Problem: Calculate trapped rainwater in elevation map.

Key Insight: Two pointers or prefix/suffix max arrays.

Pattern: Two Pointers

38. Valid Anagram (LeetCode #242) - Easy

Problem: Check if two strings are anagrams.

Key Insight: Character frequency count or sorting.

Pattern: Character Counting

39. Top K Frequent Elements (LeetCode #347) - Medium

Problem: Find k most frequent elements in array.

Key Insight: Hash map + heap or bucket sort.

Pattern: Frequency Analysis

40. Meeting Rooms II (LeetCode #253) - Medium

Problem: Find minimum meeting rooms needed.

Key Insight: Sort by start time, use min heap for end times.

Pattern: Interval Scheduling

41. Search in Rotated Sorted Array (LeetCode #33) - Medium

Problem: Search target in rotated sorted array.

Key Insight: Modified binary search - determine which half is sorted.

Pattern: Binary Search

42. Palindromic Substrings (LeetCode #647) - Medium

Problem: Count all palindromic substrings.

Key Insight: Expand around centers for each possible center.

Pattern: String Manipulation

43. Longest Palindromic Substring (LeetCode #5) - Medium

Problem: Find the longest palindromic substring.

Key Insight: Expand around centers or dynamic programming.

Pattern: String DP

Bonus Advanced Questions (7 Questions)

44. Alien Dictionary (LeetCode #269) - Hard

Problem: Determine order of characters in alien language.

Key Insight: Topological sort on character dependencies.

Pattern: Topological Sort

45. Design Add and Search Words Data Structure (LeetCode #211) - Medium

Problem: Design data structure supporting add/search with wildcards.

Key Insight: Trie with DFS for wildcard handling.

Pattern: Trie

46. Word Search (LeetCode #79) - Medium

Problem: Find if word exists in 2D board.

Key Insight: DFS backtracking on grid.

Pattern: Backtracking

47. Combination Sum (LeetCode #39) - Medium

Problem: Find all combinations that sum to target.

Key Insight: Backtracking with repetition allowed.

Pattern: Backtracking

48. Pacific Atlantic Water Flow (LeetCode #417) - Medium

Problem: Find cells where water can flow to both oceans.

Key Insight: DFS from ocean borders inward.

Pattern: Graph Traversal

49. Minimum Window Substring (LeetCode #76) - Hard

Problem: Find minimum window containing all characters of string t.

Key Insight: Sliding window with character frequency tracking.

Pattern: Sliding Window

50. Sliding Window Maximum (LeetCode #239) - Hard

Problem: Find maximum in each sliding window of size k.

Key Insight: Deque to maintain decreasing order of elements.

Pattern: Sliding Window + Deque

Study Strategy

Week 1-2: Master the Fundamentals (Questions 1-20)

Focus on arrays, strings, and basic tree problems. These form the foundation.

Week 3-4: Dynamic Programming & Graphs (Questions 21-35)

Tackle DP patterns and graph traversal algorithms.

Week 5-6: Advanced Topics (Questions 36-50)

Work on harder problems involving complex data structures and algorithms.

How InterviewCodeAssist Helps

While studying these problems is crucial, having real-time assistance during interviews can be the difference between success and failure. InterviewCodeAssist provides:

• Instant Pattern Recognition: Immediately identifies which of these 50 patterns applies to any new problem
• Real-time Hints: Provides gentle nudges when you're stuck, without giving away solutions
• Complexity Analysis: Helps you quickly determine time and space complexity
• Code Optimization: Suggests improvements to make your solution more efficient
• 100% Undetectable: Works seamlessly during screen sharing interviews

Practice Tips

1. Understand, Don't Memorize: Focus on the underlying patterns
2. Code Without IDE: Practice writing code by hand or in basic editors
3. Explain Your Thinking: Practice verbalizing your approach
4. Time Yourself: Aim to solve easy problems in 15 minutes, medium in 25 minutes
5. Review Multiple Solutions: Learn different approaches to each problem

Common Interview Patterns Summary

• Two Pointers: Arrays with sorted data or palindromes
• Sliding Window: Subarray/substring problems
• Hash Maps: Frequency counting, lookups
• DFS/BFS: Tree/graph traversal
• Dynamic Programming: Optimization problems with overlapping subproblems
• Binary Search: Sorted arrays, search space reduction
• Backtracking: Generate all possibilities
• Greedy: Local optimal choices

Ready to Ace Your Interview?

These 50 questions cover the core concepts tested in 95% of coding interviews. Combined with InterviewCodeAssist's real-time assistance, you'll be prepared for any challenge. Start practicing today and join thousands of developers who've landed their dream jobs using this proven approach.

Remember: The goal isn't to memorize solutions, but to recognize patterns and understand fundamental algorithmic concepts. With consistent practice and the right tools, you'll transform from nervous candidate to confident interviewer.