374. Guess Number Higher or Lower

题目：

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.            

答案：
二分查找思想

// 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,high=n;
        int mid;
        while(low<=high){
            mid=low+(high-low)/2;                   //直接写mid=(low+high)/2会溢出
            if(guess(mid)==0){
                return mid;
            }
            else if(guess(mid)==-1){
                high=mid-1;
            }
            else{
                low=mid+1;
            }
        }
        return mid;
    }
};