添加二分查找算法实现和测试

实现二分查找算法 binary_search，包含测试函数 test_binary_search
支持各种边界情况测试

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

main.py

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+def binary_search(arr, target):
+    """
+    二分查找算法
+    返回目标值在数组中的索引，如果不存在则返回 -1
+    """
+    left, right = 0, len(arr) - 1
+
+    while left <= right:
+        mid = left + (right - left) // 2  # 防止溢出
+
+        if arr[mid] == target:
+            return mid
+        elif arr[mid] < target:
+            left = mid + 1
+        else:
+            right = mid - 1
+
+    return -1
+
+
+def test_binary_search():
+    """测试二分查找函数"""
+    test_cases = [
+        # (数组, 目标值, 期望索引)
+        ([1, 2, 3, 4, 5], 3, 2),
+        ([1, 2, 3, 4, 5], 1, 0),
+        ([1, 2, 3, 4, 5], 5, 4),
+        ([1, 2, 3, 4, 5], 0, -1),
+        ([1, 2, 3, 4, 5], 6, -1),
+        ([], 5, -1),
+        ([5], 5, 0),
+        ([5], 3, -1),
+        ([1, 3, 5, 7, 9], 7, 3),
+        ([1, 3, 5, 7, 9], 2, -1),
+    ]
+
+    all_passed = True
+    for i, (arr, target, expected) in enumerate(test_cases):
+        result = binary_search(arr, target)
+        if result == expected:
+            print(f"测试用例 {i + 1} 通过: binary_search({arr}, {target}) = {result}")
+        else:
+            print(f"测试用例 {i + 1} 失败: binary_search({arr}, {target}) = {result}, 期望 {expected}")
+            all_passed = False
+
+    if all_passed:
+        print("所有测试用例通过!")
+    else:
+        print("部分测试用例失败!")
+
+    return all_passed
+
+
+if __name__ == "__main__":
+    # 运行测试
+    test_binary_search()