LeetCode 374. 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.

这个题好坑！
没有明白给出的API返回值，原来"My number"是指正确的那个number，错了几次才明白过来。
还有：
 mid = (low + high) / 2;

 mid = low + (high - low) / 2;

第一种计算方法会Time Limit Exceeded，原因可能是（low + high）的结果超过int的最大范围，越界。
可以使用第一种公式，但把数据改成long型。
详细分析见点击打开链接

// 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;
int high = n;
int res;
int mid;
while(high > low){
// mid = (low + high) / 2;
mid = low + (high - low) / 2;
res = guess(mid);
if(res == 1) low = mid + 1;
else if(res == -1) high = mid - 1;
else return mid;
}
return low;
}
};