```
// 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 left = 1,right=n;
while(left<right)
{
int mid = left+(right-left)/2;
int res = guess(mid);
if(res==1) left = mid+1;
else if(res==-1) right = mid-1;
else return mid;
}
return right;
}
};
```