Longest Repeating Character Replacement

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.

Update 2016-11-12

A more explainable solution:

public int characterReplacement(String s, int k) {
        int len = s.length();
        if (len == 0) {
            return 0;
        }
        
        int[] count = new int[26];
        int max = 0;
        int start = 0;
        for (int end = 0; end < len; end++) {
            max = Math.max(max, ++count[s.charAt(end) - 'A']);
            if (max + k < end - start + 1) {
                count[s.charAt(start++) - 'A']--;
            }
            
        }
        return len - start;
    }

The idea is that the window never needs to shrink. After the length of the substring expands to maximum, when we add one more char, if it is the char with the maximum occurrence, nothing changes, otherwise, the length of the substring increases but max + k doesn't, so we need to move window leftward (start++). In the following rounds, until we find another char has more occurrence, we only need to move window to keep the max length correct. So "max" here is defined as the maximum occurrence seen so far, not the maximum occurrence in current window.





This problem asks us to find the longest substring of repeating characters if we can make at most k replacement. Now if we remove the restriction that we can only make k replacement, the solution should be length of string - occurrence of character with maximum occurrence. With the k restriction, we can use sliding window manner, if length of the window - max occurrence so far > k, we know we cannot construct such substring with at most k replacement, so we move window rightward. The longest substring should be updated with the length of the window.

public int characterReplacement(String s, int k) {
        if (s.length() == 0) {
            return 0;
        }
        int start = 0;
        int len = s.length();
        int[] count = new int[26];
        int maxCount = 0;
        int rst = 0;
        for (int i = 0; i < len; i++) {
            maxCount = Math.max(maxCount, ++count[s.charAt(i) - 'A']);
            while (i - start + 1 - maxCount > k) {
                maxCount = Math.max(maxCount, --count[s.charAt(start) - 'A']);
                start++;
            }
            rst = Math.max(rst, i - start + 1);
        }
        return rst;
    }