猜想數值大了或者小了 Guess Number Higher or Lower

問題：

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.

解決：

① 使用折半查找便可。

/* 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 { // 1ms
    public int guessNumber(int n) {
        int l = 1;
        int r = n;
        while(l <= r){
            int mid = (r - l) / 2 + l;
            if(guess(mid) == 0) return mid;
            else if(guess(mid) == 1) l = mid + 1;
            else r = mid - 1;
        }
        return l;
    }
}