374. Guess Number Higher or Lower

binary search. 开始偷懒了一下mid没有写成这样：mid = l + (r - l) / 2，于是卡了很久，简直报应= =

    public int guessNumber(int n) {
        int l = 1;
        int r = n;
        int mid = l + (r - l) / 2;
        int guessAns = guess(mid);
        while(guessAns != 0) {
            if(guessAns < 0) {
                r = mid - 1;
            } else {
                l = mid + 1;
            }
            mid = l + (r - l) / 2;
            guessAns = guess(mid);
        }
        return mid;
    }