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.

Tips:

非常简单，用简单的二分法就能做，需要注意的是先算guess(n) == 0的情况，不然会超时。还有就是注意guess(n)的定义。

Code:

/* The guess API is defined in the parent class GuessGame.
   @param num, your guess
   @return -1 if my number is lower, 1 if my number is higher, otherwise return 0
      int guess(int num); */

public class Solution extends GuessGame {
    public int guessNumber(int n) {
        if (guess(n) == 0) {
            return n;
        }
        int start = 1;
        int end = n;
        while (start < end) {
            int mid = start + (end - start) / 2;
            if (guess(mid) == 0) {
                return mid;
            } else if (guess(mid) == -1) {
                end = mid;
            } else {
                start = mid;
            }
        }
        return start;
    }
}