374. Guess Number Higher or Lower

We are playing the Guess Game. The game is as follows:

I pick a number from 1 to n. You have to guess which number I picked.

Every time you guess wrong, I'll tell you whether the number is higher or lower.

You call a pre-defined API guess(int num) which returns 3 possible results (-1, 1, or 0):

-1 : My number is lower
 1 : My number is higher
 0 : Congrats! You got it!

Example:

n = 10, I pick 6.

Return 6.
题目含义：1到n里面选了一个数，让你来猜是哪一个。
如果目标数比你猜的数小，返回-1，

如果目标数比你猜的数大，返回1，
如果两数相等，返回0

    public int guessNumber(int n) {
        if (guess(n) == 0) return n;
        int left = 1,right =n;
        while (left<right)
        {
            int mid = left + (right - left) / 2;
            int t = guess(mid);
            if (t == 0 )return mid;
            else if (t>0) left=mid;
            else right = mid;
        }
        return left;        
    }