/// <summary>
        /// 二分查找
        /// </summary>
        /// <param name="arr">已经排序过的数组</param>
        /// <param name="searchkey">搜索值</param>
        /// <returns></returns>
        private static int BinarySearch(int[] arr, int searchkey)
        {
            int start = 0;
            int end = arr.Length - 1;
            int mid = 0;
            while (start <= end)
            {
                mid = (start + end) / 2;
                if (arr[mid] < searchkey) //中间值小于 搜索值,说明要查找值在尾部
                {
                    start = mid + 1;
                }
                else if (arr[mid] > searchkey)//中间值大于搜索值，说明要搜索值在首部
                {
                    end = mid - 1;
                }
                else
                {
                    return mid;
                }
            }
            return -mid;
        }