二分查找算法

递归形式

	public static int binarySearch(int[] arr, int key, int head, int end) {
		int index = -1;
		int mid = (head + end) / 2;
		
		if(key == arr[mid]) {
			return mid;
		}
		if (head >= end) {
			return -1;
		}
		if (head <= end) {
			if (key > arr[mid]) {
				index = binarySearch(arr, key, mid + 1, end);
				//因为在前面已经判断了arr[mid]和key的值是否相等，因此执行+1操作，若不执行，则会陷入死循环
			}
			if (key < arr[mid]) {
				index = binarySearch(arr, key, head, mid - 1);
			}
		}
		
		return index;
	}

非递归形式

	public static int binarySearch(int[] arr, int key, int head, int end) {
		int index = -1;
		int mid;
		
		mid = (head + end) / 2;
		while (head <= end) {
			if (arr[mid] == key) {
				index = mid;
				return index;
			}
			if (arr[mid] < key) {
				head = mid + 1;
				mid = (head + end) / 2;
			}
			if (arr[mid] > key) {
				end = mid - 1;
				mid = (head + end) / 2;
			}
		}
		
		
		return index;
	}