Longest Repeating Character Replacement

Problem

Given a string that consists of only uppercase English letters, you can replace any letter in the string with another letter at most k times. Find the length of a longest substring containing all repeating letters you can get after performing the above operations.

Note: Both the string's length and k will not exceed 104.

Example 1:

Input:
s = "ABAB", k = 2

Output:
4

Explanation:
Replace the two 'A's with two 'B's or vice versa.

Example 2:

Input:
s = "AABABBA", k = 1

Output:
4

Explanation:
Replace the one 'A' in the middle with 'B' and form "AABBBBA".
The substring "BBBB" has the longest repeating letters, which is 4.

Solution

Sliding Window Solution

public class Solution {
    public int characterReplacement(String s, int k) {
        if (s == null || s.length() == 0) return 0;
        int[] count = new int[128];
        int start = 0, max = 0;
        for (int end = 0; end < s.length(); end++) {
            max = Math.max(max, ++count[s.charAt(end)]);
            if (end - start + 1 - max > k) count[s.charAt(start++)]--;
        }
        return s.length() - start;
    }
}

Analysis

This is a typical sliding window solution, although it's not very straightforward
The key idea is move the sliding window when end - start + 1 - max > k
That means we have met a largest possible sequence, we can move the window to right
At the end, we return the sequence we have passes, which is s.length() - start
Notice that we collect the count in place