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.

 

/* 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 (n == 1) return 1;
        int beg = 1, end = n;
        while (beg + 1 < end) {
            int mid = beg + (end - beg) / 2;
            int res = guess(mid);
            if (res == 0) {
                return mid;
            } else if (res > 0) {
                beg = mid;
            } else {
                end = mid;
            }
        }
        if (guess(beg) == 0) {
            return beg;
        } else {
            return end;
        }
    }
}