java二分查找

public int binarySearch(int[] arr,int key){

  if (arr == null) {

    return -1;

  }

  int length = arr.length;

  if (length == 0) {

    return -1;

  }

  int mid = length/2;

  if (arr[mid] == key) {

    return mid;

  }

  int start = 0;

  int end = length-1;

  while (start <= end) {

    mid = (end - start)/2+start;

    if (arr[mid] < key) {

      start = mid +1;

    } else if (arr[mid]>key) {

      end = mid -1;

    }else{

      return mid;

    }

  }

  return -1;

}