二分查找法

1. 循环

 public static int demo(int[] arr,int num){
        int start=0;
        int end=arr.length;
        while (true){
            int mid = (start+end)/2;
            if(start>end){
                return -1;
            }
            if(arr[mid]==num){
                return mid;
            }else if(arr[mid]>num){
                end = mid-1;
            }else if(arr[mid]<num){
                start = mid+1;
            }
        }
    }