Longest Repeating Character Replacement

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Problem

Given a string s consisting of uppercase letters and an integer k, you may change up to k characters in the string to any other uppercase letter. Return the length of the longest substring containing a single repeated letter you can obtain after performing these changes.

Examples

Example 1

Input:s = "ABAB", k = 2

Output:4

Example 2

Input:s = "AABABBA", k = 1

Output:4

Constraints

1 <= s.length <= 10^5
0 <= k <= s.length

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How to approach it: the Sliding Window pattern

A window — a contiguous subarray or substring — expands and shrinks as it moves across the input, so you track a running condition instead of recomputing it from scratch for every possible window.

Look for this pattern when

The problem asks for the longest/shortest/best contiguous subarray or substring meeting a condition.
Brute force would recompute a sum or count for every possible window — a sign that work can be shared between overlapping ranges.

Read the full Sliding Window guide →

Video walkthroughs

Original problem on LeetCode ↗