136.Guess Number Higher or Lower

题目：

We are playing the Guess Game. The game is as follows:

我们正在玩猜猜游戏。 游戏如下：

I pick a number from 1 to n. You have to guess which number I picked.

我从1到n中选择一个数字。 你必须猜测我选择了哪个号码。

Every time you guess wrong, I'll tell you whether the number is higher or lower.

每次你猜错了，我都会告诉你这个数字是高还是低。

You call a pre-defined API guess(int num) which returns 3 possible results (-1, 1, or 0):

您调用预定义的API guess（int num），它返回3个可能的结果（-1,1或0）：

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

Example :

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

解答：

/* 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 i = 1, j = n;
        while(i < j) {
            int mid = i + (j - i) / 2;
            if(guess(mid) == 0) {
                return mid;
            } else if(guess(mid) == 1) {
                i = mid + 1;
            } else {
                j = mid;
            }
        }
        return i;
    }
}

详解：

二分搜索法