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 (-11, or 0):

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


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) {
        int start=1;
        int end=n;
            int mid = start + (end-start)/2;
            if(guess(mid) ==0){
                return mid;
            else if(guess(mid) ==-1){
                end =mid;
                start = mid+1;
        return start;