374. Guess Number Higher or Lowe

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.
猜数游戏，要求返回指定的数字，可调用guess(n)查看所猜的数字n与指定数字的大小关系。

思路
二分查找

// 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 low=1;
        while(low<=n){
            int mid=low+(n-low)/2;       //防止溢出
            int res=guess(mid);
            if(res==0)  return mid;
            else if(res==-1)    n=mid-1;
            else    low=mid+1;
        }
        return -1;
    }
};