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.

本题相对来说比较简单，代码如下：

/* 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 low = 1;
        int high = n;
        while(low<=high) {
            int mid = low + ( high - low ) / 2;
            if(guess(mid)==0) return mid;
            else if(guess(mid)>0) low = mid+1;
            else high = mid-1;
        }
        return -1;
    }
}
//the run time complexity could be O(logn), the space complexity could be O(1);