What is the time complexity of LeetCode 424 solution?

O(n). Each pointer moves at most n times.

Why use a hash map instead of array for frequencies?

Array's fine (ord(c)-65), but dict's Pythonic and flexible.

Can this solve with DP?

Yes, O(n^2k)—TLE city. Window wins.

Sliding Through Chaos: Mastering LeetCode 424's Character Replacement Puzzle

Picture this: a string of letters twisting like a DNA strand, and you've got k swaps to forge the longest uniform run. LeetCode 424 drops you right in—here's the vivid trace that makes it click.

theAIcatchup Apr 09, 2026 3 min read

Animated visual trace of sliding window expanding and contracting on LeetCode 424 example string

⚡ Key Takeaways

Sliding window + live max_freq tracks longest k-replaceable uniform substring in linear time. 𝕏
No shrink loop needed—single left++ per violation suffices due to incremental adds. 𝕏
Preps for AI sequence processing; think token windows in LLMs or bio strings. 𝕏

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#LeetCode #algorithm visualization #algorithm-tracing #algorithms #character-replacement #coding interview #interview-prep #leetcode-424 #sliding window #two pointers

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⚡ Key Takeaways

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