leetcode 374. Guess Number Higher or Lower

二分。

    int guessNumber(int n) {
        int start = 1;
        int end = n;
        while (start <= end) {
            int mid = start + (end - start) / 2;
            int res = guess(mid);
            if (!res)
                return mid;
            if (res == -1)
                end = mid - 1;
            else
                start = mid + 1;
        }
    }