# Time: O(N^2)
# Space: O(N)

from collections import Counter

class Solution:
    def characterReplacement(self, s: str, k: int) -> int:
        '''
        Sliding window approach
        '''
        
        def window_size(l, r):
            return r - l + 1
        
        # Need to keep track of frequency of each character in the
        # window
        count = Counter()
        l = r = 0
        result = 0
        
        while r < len(s):
            # Increment the frequency of the `r`th char
            count.update(s[r]) 
            
            # We can perform upto K replacements. So we prefer least num
            # of replacements (<= K). In the current window, we can perform
            # least replacements for the most common character in the range.
            #
            # e.g. AAABB K=2, in range 0 to 4, most common char is A, count=3
            # so it just needs 5 - 3 = 2 replacements which is <= K which is ok.
            #
            # But, if K = 1, then we need 5 - 2 = 2 replacements for the same range
            # but 2 > K so this window is not valid. We need to move left pointer in
            # this case and reduce its count.
            if window_size(l, r) - count.most_common(1)[0][1] > k:
                count.subtract(s[l])
                l += 1
            
            result = max(result, window_size(l, r))
            r += 1
        
        return result