Add solution for 'longest repeating character replacement'

Implement a sliding window approach for solving the problem, ensuring efficient handling of character replacement within a string. Included unit tests to validate functionality and edge cases.

Author: nicholasadamou

Sliding Window/longest_repeating_character_replacement.py

@@ -0,0 +1,29 @@
+# https://neetcode.io/problems/longest-repeating-substring-with-replacement
+
+import unittest
+
+from collections import Counter
+
+def character_replacement(s: str, k: int) -> int:
+    counts = Counter(s)
+
+    left = 0
+    result = 0
+
+    for right, char in enumerate(s):
+        # 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:
+            left += 1
+
+        result = max(result, right - left + 1)
+
+    return result
+
+class Test(unittest.TestCase):
+    def test_character_replacement(self):
+        self.assertEqual(character_replacement("XYYX", 2), 4)
+        self.assertEqual(character_replacement("AAABABB", 1), 5)