📌 Introduction

When it comes to LeetCode, it’s not about solving hundreds of problems.
It’s about solving the right ones — the kind that teach you reusable patterns, not just solutions.

Whether you’re prepping for interviews at top tech companies, building a strong foundation in DSA, or brushing up on problem-solving skills, this list of 25 curated problems will cover the essential patterns you must master.

🔍 Why These 25 Questions?

These problems aren’t just random selections — they’re categorized by topic and chosen to help you:

• Recognize problem-solving patterns
• Improve time-space complexity intuition
• Build confidence with recursion, DP, trees, graphs, and more

Let’s dive into them.

📂 Arrays

Learn to apply two pointers, prefix products, and interval logic.

• 3Sum
• Combination Sum
• Merge Intervals
• Insert Interval
• Product of Array Except Self

🧵 Strings

String manipulation teaches you sliding window, hash maps, and pattern recognition:

• Longest Substring Without Repeating Characters
• Minimum Window Substring

🧱 Stacks

Understanding stacks unlocks problems related to expression evaluation, histogram logic, and backtracking:

• Trapping Rain Water
• Evaluate Reverse Polish Notation
• Min Stack

⚖️ Heaps / Priority Queues

These are powerful for top-k problems, greedy strategies, and simulations:

•  Find Median from Data Stream
• K Closest Points to Origin

🔍 Binary Search

Binary search isn’t just for sorted arrays — it’s a pattern for narrowing search spaces:

• Search in Rotated Sorted Array
• Time Based Key-Value Store

🌳 Binary Trees

Binary tree problems are critical for recursive thinking, DFS, and BFS:

• Binary Tree Level Order Traversal
• Lowest Common Ancestor of a Binary Tree
• Serialize and Deserialize Binary Tree

🧮 Dynamic Programming (DP)

DP questions test your ability to identify overlapping subproblems and optimal substructure:

Maximum Subarray

Coin Change

🌐 Graphs

Graph problems span BFS, DFS, cycle detection, and topological sorting:

•  Clone Graph
•  Number of Islands
• Course Schedule
• 01 Matrix
• Rotting Oranges

🔁 Recursion / Backtracking

Learn how to explore the solution space recursively:

 Permutations

📚 Top 5 Books to Strengthen Your DSA Fundamentals

Want to go beyond problem-solving and dive into concepts? Start with these books:

1. Grokking Algorithms – Aditya Bhargava (click here to buy on Amazon )
   Friendly and visual — a perfect beginner’s guide.
2. Data Structures and Algorithms Made Easy – Narasimha Karumanchi (click here to buy on Amazon )
   Interview-oriented, with clear explanations and real-world problems.
3. Problem Solving with Algorithms and Data Structures using Python – Miller & Ranum (click here to buy on Amazon)
   Python-based DSA walkthroughs with hands-on examples.
4. Introduction to Algorithms (CLRS) – Cormen, Leiserson, Rivest, Stein (click here to buy on Amazon)
   The gold standard — in-depth and widely referenced.

👣 Final Thoughts

Success in technical interviews doesn’t come from quantity — it comes from strategy. These 25 LeetCode problems represent the core concepts and patterns that show up again and again.

🛠 Solve them deeply.
🔁 Revisit them regularly.
🧠 Understand the why — not just the how.

Let quality guide your practice.