We use a sliding window. A window is valid if (window_size - max_freq_char) <= k.

1. count = {}, max_f = 0, l = 0, res = 0.
2. For r from 0 to len(s)-1:
   • count[s[r]] = count.get(s[r], 0) + 1.
   • max_f = max(max_f, count[s[r]]).
   • If (r - l + 1) - max_f > k:
     • count[s[l]] -= 1.
     • l += 1.
   • res = max(res, r - l + 1).
3. Return res.

• Time Complexity: O(N).
• Space Complexity: O(1) (26 characters).

def character_replacement(s, k):
    count = {}
    res = 0
    l = 0
    max_f = 0
    
    for r in range(len(s)):
        count[s[r]] = 1 + count.get(s[r], 0)
        max_f = max(max_f, count[s[r]])
        
        while (r - l + 1) - max_f > k:
            count[s[l]] -= 1
            l += 1
            
        res = max(res, r - l + 1)
        
    return res