Guess Number Higher or Lower

We are playing the Guess Game. The game is as follows:

I pick a number from 1 to n. You have to guess which number I picked.

Every time you guess wrong, I’ll tell you whether the number is higher or lower.

You call a pre-defined API guess(int num) which returns 3 possible results (-1, 1, or 0):

-1 : My number is lower
1 : My number is higher
0 : Congrats! You got it!
Example:
n = 10, I pick 6.

Return 6.

此题题意不清，My number指的是最终的答案数字。

class Solution {public:    int guessNumber(int n) {        int left = 1, right = n ;        while(left <= right){            int middle = left + (right - left) / 2;            int index = guess(middle);            if(index == -1){                right = middle -  1 ;            }            else if(index == 1){                left = middle + 1;            }             else                return middle;        }        return -1;    }};