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.

题意：

猜数字，有一个内置的函数guess，如果目标数字大于我们猜的，就返回1，小于则返回-1，相等就返回0.

思路：

二分法。设置l和r，初始值分别为1，n。不取0是因为题目说明1-n，并且注意题目说的my number是指目标数字。

代码：

/* 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,r=n;
        while(l<=r)
        {
            int t=l+(r-l)/2;
            if(guess(t)==1)
                l=t+1;
            else if(guess(t)==-1)
                r=t-1;
            else
                return t;
        }
        return -1;
    }
}