代码如下：

   public static int BSearch(int[] arr, int start, int end, int target) {if (arr == null) {return -1;}if (start > end) {return -1;}int mid = (start + end) / 2;if(arr[mid]==target){return mid;} else if (arr[mid] > target) {return BSearch(arr, start, mid - 1, target);}else {return BSearch(arr, mid + 1, end, target);}