LeetCode 374. Guess Number Higher or Lower

class Solution {
public:
    int guessNumber(int n) {
        int l = 1, r = n;
        while (l < r) {
            int mid = (r - l) / 2 + l;
            int ans = guess(mid);
            if (ans == -1) r = mid - 1;
            else if (ans == 1) l = mid + 1;
            else return mid;
        }
        return r;
    }
};