Sliding Window/Longest Repeating Character ReplacementMedium

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Longest Repeating Character Replacement

Sliding Window·Medium

You are given a string s consisting of uppercase English letters and an integer k. You may replace at most k characters in s with any uppercase English letter.

Return the length of the longest substring that can be made to contain only one repeated character after performing at most k replacements.

The classic sliding-window invariant: a window of length L is valid iff L - max_freq <= k, where max_freq is the count of the most frequent character inside the window.

Examples

Input: s = "ABAB", k = 2

Output: 4

Replace the two `A`s (or two `B`s) to obtain `"BBBB"` or `"AAAA"`.

Input: s = "AABABBA", k = 1

Output: 4

Replace the middle `"A"` in `"ABAB"` to get `"ABBB"`, length 4.

Constraints

1 <= s.length <= 10^5
s consists of uppercase English letters only.
0 <= k <= s.length

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Tests

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Passed32 ms

First attempt — retry to track metric movement here.

Your approach

Pattern:Sliding Window
Time:O(N)
Space:O(1)

Complete approach — pattern and complexity both named.

Coach

What you got right

You correctly matched the sorted input requirement and implemented a linear-time scan using two pointers.

Where it diverged

No divergence detected. The code correctly aligns with the stated approach.

Next attempt: focus on

Try solving related sliding window problems to build familiarity with two-pointer variants.

Signals

No signals fired — clean run, you stayed in flow.