Problem URL: Replace the substring for balanced string

We will use a sliding window to keep track of the number of characters in the current substring. We will keep track of the number of characters we need to replace. We will keep track of the minimum length of the substring we need to replace.

class Solution:
    def balancedString(self, s: str) -> int:
        count = collections.Counter(s)
        res = n = len(s)
        i = 0
        for j, c in enumerate(s):
            count[c] -= 1
            while i<n and all(n/4 >= count[c] for c in 'QWER'):
                res = min(res, j-i+1)
                count[s[i]] += 1
                i += 1
        return res

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