10 Essential Design Problems for DSA Interviews

These days, interviewers are increasingly testing design and implementation skills rather than just algorithmic problem-solving. If you're preparing for interviews, make sure you're comfortable with these problems!

1. LRU Cache

LeetCode 146

Build a cache that evicts the least recently used item when full. You need O(1) for both get and put, which means combining a hash map with a doubly linked list.

Follow-ups:

• LFU Cache (LC 460)
• Design Most Recently Used Queue (LC 1756)

---

2. Min Stack

LeetCode 155

Implement a stack that can return its minimum element in O(1) time. Simple but elegant - teaches you to maintain auxiliary information efficiently.

Follow-ups:

• Max Stack (LC 716)
• Try implementing without extra space by encoding min with each element

---

3. Implement Trie (Prefix Tree)

LeetCode 208

Build the foundation for autocomplete and spell checkers. A tree where each node represents a character, perfect for prefix-based operations.

Follow-ups:

• Design Add and Search Words (LC 211) - supports wildcards
• Word Search II (LC 212) - uses Trie for optimization
• Replace Words (LC 648)
• Implement Magic Dictionary (LC 676)

---

4. Design HashMap

LeetCode 706

Build a hash map from scratch without using built-in hash tables. You'll handle hash functions, collision resolution, and understand what's happening under the hood.

Follow-ups:

• Design HashSet (LC 705)
• Implement with open addressing instead of chaining
• Add resizing when load factor exceeds threshold

---

5. Insert Delete GetRandom O(1)

LeetCode 380

Support insert, delete, and get random element - all in O(1). The trick is combining an ArrayList for random access with a HashMap for O(1) lookup. The delete operation is beautifully clever.

Follow-ups:

• Insert Delete GetRandom O(1) - Duplicates allowed (LC 381)

---

6. Design Twitter

LeetCode 355

Build a simplified Twitter with post, follow, unfollow, and timeline features. Tests your ability to model real-world systems and efficiently merge sorted lists from multiple users.

---

7. Design a Stack With Increment Operation

LeetCode 1381

A stack where you can increment the bottom k elements. Naive approach is O(k), but lazy propagation makes it O(1). Great introduction to deferred operations.

Follow-ups:

• Think about how databases use similar techniques for deferred constraint checking

---

8. Time Based Key-Value Store

LeetCode 981

Store versioned values with timestamps and retrieve the value at or before any timestamp. Teaches you about versioning systems and binary search on historical data.

Follow-ups:

• Design In-Memory File System (LC 588)
• Think about how Git stores versioned data

---

9. Design Circular Queue

LeetCode 622

Implement a queue with fixed size using an array. The wraparound logic and distinguishing between full and empty states is trickier than it looks.

Follow-ups:

• Design Circular Deque (LC 641)
• Design Front Middle Back Queue (LC 1670)

---

10. Snapshot Array

LeetCode 1146

Support set, snap, and get operations where snap creates a version checkpoint. The key is not copying the entire array - only store what changed in each snapshot.

---

Bonus Problems Worth Checking Out

• Design Browser History (LC 1472)
• Design Underground System (LC 1396)
• Design Phone Directory (LC 379)
• Design Search Autocomplete System (LC 642)
• All O'one Data Structure (LC 432)
• Design Authentication Manager (LC 1797)
• Serialize and Deserialize Binary Tree (LC 297)

---

Add few more best problems in the comments below !!

Good luck with your interviews! 🚀