Optimize sliding window logic to track max character frequency.

Replaced `Counter` with `defaultdict` for efficiency and added a `max_frequency` variable to keep track of the most frequent character in the current window. Updated the window shrinking condition to use `max_frequency`, improving clarity and performance. Added additional test cases for better coverage.

Author: nicholasadamou

Sliding Window/longest_repeating_character_replacement.py

@@ -2,21 +2,26 @@
 
 import unittest
 
-from collections import Counter
+from collections import defaultdict
 
 def character_replacement(s: str, k: int) -> int:
-    counts = Counter(s)
+    counts = defaultdict(int)
 
     left = 0
+    max_frequency = 0 # Track the frequency of the most frequent character in the current window
     result = 0
 
     for right, char in enumerate(s):
+        counts[char] += 1
+        max_frequency = max(max_frequency, counts[char])
+
         # Shrink the window when the current window size minus the count of the most
         # frequent character in the window exceeds k. This means we have more characters
         # to replace than allowed by k, so we need to move the left pointer to reduce
         # the window size.
 
-        while (right - left + 1) - counts[char] > k:
+        while (right - left + 1) - max_frequency > k:
+            counts[s[left]] -= 1
             left += 1
 
         result = max(result, right - left + 1)
@@ -27,3 +32,5 @@ class Test(unittest.TestCase):
     def test_character_replacement(self):
         self.assertEqual(character_replacement("XYYX", 2), 4)
         self.assertEqual(character_replacement("AAABABB", 1), 5)
+        self.assertEqual(character_replacement("ABABBA", 2), 5)
+        self.assertEqual(character_replacement("AABACCC", 2), 5)