Guess Number Higher or Lower -- LeetCode

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.

思路：二分搜索

// Forward declaration of guess API.
// @param num, your guess
// @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
int guess(int num);

class Solution {
public:
    int guessNumber(int n) {
        int left = , right = n;
        while (left <= right) {
            int mid = left + (right - left) / ;
            //if the num < mid
            if (guess(mid) < )
                right = mid - ;
            //else if the num > mid
            else if (guess(mid) > )
                left = mid + ;
            else return mid;
        }
        return -;
    }
};