（二）二分查找

 前提：有序

  public  int binarySearch(int[] array,int low,int high,int key)
	{
		int mid=0;
		
		while(low<=high)
		{
			mid=(low+high)/2;
			if(array[mid]>key)
			{
				high=mid-1;
			}
			else if(array[mid]<key)
			{
				low=mid+1;
			}else
			{
				return mid;
			}
		}
		
		return -1;
		
	}