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// 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, mid, guess_result;
        while(left < right) {
            mid = left + (right - left)/2;
            guess_result = guess(mid);
            if (guess_result == 0) {
                return mid;
            } else if (guess_result == 1) {
                left = mid + 1;                
            } else {
                right = mid - 1;
            }
        }
        return right;
    }
};