[LeetCode 374] Guess Number High

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 :

Input: n = 10, pick = 6
Output: 6

Solution : Binary Search

/* 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 <= 0) {
            return 0;
        }
        
        int start = 1;
        int end = n;
        
        while (start + 1 < end) {
            int middle = start + (end - start) / 2;
            
            if (guess (middle) == 0) {
                return middle;
            }
            
            if (guess (middle) == 1) {
                start = middle + 1;
            } else if (guess (middle) == -1) {
                end = middle - 1;
            }
        }
        
        if (guess (start) == 0) {
            return start;
        } else if (guess (end) == 0) {
            return end;
        }
        
        return 0;
    }
}