1234. Replace the Substring for Balanced String

class Solution:
    def balancedString(self, s: str) -> int:
        """
        tc O(N)  sc O(1) since only cnt 4 char QWER
        note,only 4 types of char
        main idea: two pointer--- instead of checking if substring within the window is balanced, we check substring count outside window if by adding them into window (replaced into desired character) ===> as only as we make substring's cnt in the window all have spare space to allow substring outside window to add in.   
        
        e.g. 
        n = 12   n/4 = 3
        WWQQRRRRQRQQ
          l   r
          ^   ^
          | 4 |
        """
        n = len(s)
        k = n//4
        l = 0
        res = n
        d = {}
        cnt = Counter(s)
        for r in range(n):
            cnt[s[r]] -= 1
            while l < n and all(cnt[c]<=k for c in 'QWER'):
                res = min(res,r-l+1)
                cnt[s[l]] += 1 
                l += 1 
        return res