二分法

 public int BinarySearch(int[] list,int key,int low,int hight)
             {
             low = 0;
             hight = list.Length - 1;
            int mid = (low + hight) / 2;
            if (low > hight)

            { return -1; }

            else

            {
                if (key == list[mid])
                    return mid;
                else if (key < list[mid])
                { 
                   return BinarySearch(list,key,low,mid-1);
                }
                else
                {
                   return BinarySearch(list, key, mid + 1, hight);
                }
            
            }
        }
        
        }