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.

猜数字，如果正确数字比猜测数字大返回1，小返回-1，正确返回0.求正确数字。

很简单的二分查找问题。

// 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 begin = 1;
        int end = n;
        int mid = begin + (end-begin)/2;
        while(1){
            if(guess(mid)==1){
                begin = mid+1;
                mid = begin + (end-begin)/2;
            }
            else if(guess(mid)==-1){
                end = mid -1;
                mid = begin + (end-begin)/2;
            }
            else if(guess(mid)==0){
                return mid;
            }
        }
    }
};