LeetCode 219: Contains Duplicate II — Clean & Optimal Solution (C++/Python)

In this video, we solve LeetCode Problem 219: Contains Duplicate II, a very popular problem involving HashMaps, Sliding Window, and efficient duplicate detection.

This problem is excellent for beginners because it teaches you how to use hashing to track indices efficiently and how to turn a brute-force O(n²) approach into an optimal O(n) solution.

🧠 Problem Logic

We are given an array nums and an integer k.
We must determine:

Does there exist two equal elements such that the absolute difference of their indices is ≤ k?

✔️ Efficient Approach (HashMap / Unordered Map)

Keep a map that stores: value → last index where it appeared

For each element:

If it exists in the map already:

Check if i - last_index ≤ k

If yes → we found the answer (return true)

Update the index in the map

This gives us:

⏱️ Time Complexity: O(n)
📦 Space Complexity: O(n)

🔗 Problem Link

https://leetcode.com/problems/contains-dup...

👋 About Me

Hi, I'm Ajmain Fayek Diganta, a CSE student at BUET.
I upload tutorials on Competitive Programming, LeetCode, Algorithms, and C++ problem solving to help students and beginners grow.

🔗 GitHub: https://github.com/DIGANTA100

🔗 LinkedIn: https://www.linkedin.com/in/ajmain-fayek-d...

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