What is LeetCode 424 Longest Repeating Character Replacement?

It's a medium sliding window problem: max length of substring you can make uniform with <=k char swaps.

How to solve LeetCode 424 with sliding window?

Use left/right pointers, hashmap for freqs, shrink when (window_len - max_freq) > k. O(n) magic.

Does LeetCode 424 use O(1) space?

Yes — 26 letters max, dict's fine, but array[26] even tighter.

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LeetCode 424's Sliding Window Secret: Replacing Characters to Win Interviews

Q: How to solve LeetCode 424 with sliding window?

Use left/right pointers, hashmap for freqs, shrink when (window_len - max_freq) > k. O(n) magic.

Your next Big Tech interview hinges on tricks like LeetCode 424. This sliding window hack turns a nasty string puzzle into pure interview gold, letting you swap k characters for the longest uniform run.

theAIcatchup Apr 09, 2026 5 min read

Visual step-by-step trace of sliding window on LeetCode 424 string example

⚡ Key Takeaways

Sliding window turns O(n^2) brute force into O(n) elegance by maintaining live frequencies. 𝕏
Stale max_freq is a clever optimization — conservative validity check without recompute. 𝕏
Master this for 50+ similar LeetCode problems and real-world string processing in interviews. 𝕏

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#LeetCode 424 #Python #algorithm visualization #algorithms #character replacement #coding interviews #sliding window #two pointers

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Originally reported by Dev.to

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