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

from collections import Counter

class Solution:
    def characterReplacement(self, s: str, k: int) -> int:
        '''
        Sliding window approach, optimized
        '''
        
        def window_size(l, r):
            return r - l + 1
        
        # Need to keep track of frequency of each character in the
        # window
        count = Counter()
        
        # We can optimize the prev sliding window approach by just keeping
        # track of maximum frequency/count of an element we've seen so far.
        maxf = 0
        
        l = r = 0
        result = 0
        
        while r < len(s):
            # Increment the frequency of the `r`th char
            count.update(s[r]) 
            
            # If right's frequency is bigger, make it the new
            # max frequency
            maxf = max(maxf, count[s[r]])
            
            # Refer to neetcode: https://www.youtube.com/watch?v=gqXU1UyA8pk
            # TODO: Document this approach my own way
            if window_size(l, r) - maxf > k:
                count.subtract(s[l])
                l += 1
            
            result = max(result, window_size(l, r))
            r += 1
        
        return result