/// <summary>
/// 二分查找
/// </summary>
/// <param name="arr"></param>
/// <param name="low">開始索引 0</param>
/// <param name="high">結束索引 </param>
/// <param name="key">要查找的對象</param>
/// <returns></returns>
public static int BinarySearch(int[] arr, int low, int high, int key)
{
    int mid = (low + high) / 2;
    if (low > high)
        return -1;
    else
    {
        if (arr[mid] == key)
            return mid;
        else if (arr[mid] > key)
            return BinarySearch(arr, low, mid - 1, key);
        else
            return BinarySearch(arr, mid + 1, high, key);
    }
}

int[] y = new int[] {1,2,3,4,5,6,7,8,9,10,11,12,13 };
int rr = BinarySearch(y, 0, y.Length - 1, 13);
Console.Write(rr);    //12