Top 20 Coding Problems Set

Problem buckets

Arrays and strings

• Two Sum
• Longest Substring Without Repeating Characters
• Product of Array Except Self

Sliding window and pointers

• Best Time to Buy and Sell Stock
• Minimum Size Subarray Sum
• 3Sum

Trees and graphs

• Binary Tree Level Order Traversal
• Lowest Common Ancestor of BST
• Number of Islands

Dynamic programming

• Climbing Stairs
• House Robber
• Coin Change

How to practice this set

1. Solve each problem in 35 to 45 minutes.
2. Write one brute force and one optimized solution.
3. Keep mistakes in a note file.
4. Revisit weak patterns every 3 days.

Interview simulation checklist

• Explain approach before coding.
• State complexity without being asked.
• Test with edge cases.
• Refactor variable names for readability.