Longest Repeating Character Replacement

LeetCode Q 424 - 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.

Solution : Sliding Window

The problem says that we can make at most k changes to the string (any character can be replaced with any other character).

1. First, let’s say there were no constraints like the k. Given a string convert it to a string with all same characters with minimal changes. The answer to this is
   length of the entire string - number of times of the maximum occurring character in the string.

2. Given this, we can apply the at most k changes constraint and maintain a sliding window such that
   length of substring - number of times of the maximum occurring character in the substring) <= k

Code:

public int characterReplacement(String s, int k) {
	if (s == null || s.length() == 0) return 0;

	int[] count = new int[26];
	int start = 0, end = 0, maxCount = 1, res = 1;

	while (end < s.length()) {
		count[s.charAt(end) - 'A']++;
		maxCount = Math.max(maxCount, count[s.charAt(end) - 'A']);
		while (end - start + 1 - maxCount > k) {
			count[s.charAt(start++) - 'A']--;
			for (int i = 0; i < 26; i++) // update maxCount
				if (count[i] > maxCount) maxCount = count[i];
		}
		res = Math.max(res, end - start + 1);
	}

	return res;
}