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

class Solution {
public:
    int guessNumber(int n) {
        int lo=1, hi=n;
        while(lo<hi){
            int mid=lo+(hi-lo)/2;
            if(guess(mid)==0)
                return mid;
            guess(mid)==1?lo=mid+1:hi=mid-1;
        }
        return lo;
    }
};