Add Bulls and Cows solution with tests and complexity analysis

problems/medium/bulls_and_cows_299/solution.py

@@ -0,0 +1,26 @@
+# Tags: String, Hash Table, Counting
+from collections import Counter
+
+
+class Solution:
+    def get_hint(self, secret: str, guess: str) -> str:
+        """
+        Time complexity: O(n), where n is the length of secret/guess.
+        We traverse the strings once to count bulls and build counters,
+        then iterate through at most 10 digits (0-9) to count cows.
+
+        Space complexity: O(1), counters store at most 10 digits (0-9).
+        """
+        bulls = 0
+        cows = 0
+        s_count = Counter()
+        g_count = Counter()
+        for s, g in zip(secret, guess):
+            if s == g:
+                bulls += 1
+            else:
+                s_count[s] += 1
+                g_count[g] += 1
+        for digit in s_count:
+            cows += min(s_count[digit], g_count[digit])
+        return f"{bulls}A{cows}B"

problems/medium/bulls_and_cows_299/test_solution.py

@@ -0,0 +1,14 @@
+import pytest
+from .solution import Solution
+from tests.base_test import BaseTestSolution
+
+
+class TestSolution(BaseTestSolution):
+    solution = Solution()
+
+    @pytest.mark.parametrize("method_name, secret, guess, expected, timeout", [
+        ('get_hint', "1807", "7810", "1A3B", BaseTestSolution.DEFAULT_TIMEOUT),
+        ('get_hint', "1123", "0111", "1A1B", BaseTestSolution.DEFAULT_TIMEOUT),
+    ])
+    def test_get_hint(self, method_name, secret, guess, expected, timeout):
+        self._run_test(self.solution, method_name, (secret, guess,), expected, timeout)