Replace the Substring for Balanced String

Problem

You are given a string s of length n containing only four kinds of characters: 'Q', 'W', 'E', and 'R'.

A string is said to be balanced** **if each of its characters appears n / 4 times where n is the length of the string.

Return **the minimum length of the substring that can be replaced with *any* other string of the same length to make s balanced**. If s is already *balanced*, return 0.

Example 1:

Input: s = "QWER"
Output: 0
Explanation: s is already balanced.

Example 2:

Input: s = "QQWE"
Output: 1
Explanation: We need to replace a 'Q' to 'R', so that "RQWE" (or "QRWE") is balanced.

Example 3:

Input: s = "QQQW"
Output: 2
Explanation: We can replace the first "QQ" to "ER".

Constraints:

n == s.length
4 <= n <= 10^5
n is a multiple of 4.
s contains only 'Q', 'W', 'E', and 'R'.

Solution (Java)

class Solution {
    public int balancedString(String s) {
        int n = s.length();
        int ans = n;
        int excess = 0;
        int[] cnt = new int[128];
        cnt['Q'] = cnt['W'] = cnt['E'] = cnt['R'] = -n / 4;
        for (char ch : s.toCharArray()) {
            if (++cnt[ch] == 1) {
                excess++;
            }
        }
        if (excess == 0) {
            return 0;
        }
        int i = 0;
        int j = 0;
        while (i < n) {
            if (--cnt[s.charAt(i)] == 0) {
                excess--;
            }
            while (excess == 0) {
                if (++cnt[s.charAt(j)] == 1) {
                    excess++;
                }
                ans = Math.min(i - j + 1, ans);
                j++;
            }
            i++;
        }

        return ans;
    }
}

Explain:

nope.

Complexity:

Time complexity : O(n).
Space complexity : O(n).