LeetCode: Guess Number Higher or Lower

LeetCode: 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):

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

Example:

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n = 10, I pick 6.

Return 6.
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/* 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 l = 1;
        int r = n;
        while (l <= r) {
            int m = l+ (-l + r) / 2;
            int res = guess(m);
            if (res == 0) {
                return m;
            }

            if (res < 0) {
                r = m - 1;
            } else {
                l = m + 1;
            }
        }
         return -1;
    }
}