374. Guess Number Higher or Lower

Problem:

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.

Analysis:

The condition is very clear, so use double check template

Solution:

/* 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) {
        int left = 0, right  = n;
        while (left + 1 < right) {
            int mid = left + (right - left) / 2;
            if (guess(mid) == 0)
                return mid;
            else if (guess(mid) == -1)
                right = mid;
            else 
                left = mid;
        }       
        return guess(left) == 0 ? left : right;
    }
}