折半查找

	/* 折半查找  */
		/* 输出：9 */
		static int BinarySearch(int [] a, int n, int key){
			int low, high, mid;
			low = 0;
			high = n;
			while(low <= high){
				mid = (low + high) / 2; /* 折半  */
				if (key < a[mid]){
					high = mid - 1;
				}
				else if (key > a[mid]){
					low = mid + 1;
				}
				else 
					return mid;
			}
			return 0;
		}