[leetcode]374. Guess Number Higher or Lower

这是leetcode第374题

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!

递归二分查找

看完题目，我的第一想法是，递归二分查找。于是很快就写出如下代码。
Submit得到结果StackOverFlow，说明递归太深了。

public class Solution extends GuessGame {
    public int guessNumber(int n) {
        return guessNumberInternal(1, n);
    }
    
    private int guessNumberInternal(int left, int right) {
        int middle = (left + right) / 2;
        int result = guess(middle);
        switch (result) {
            case 1:
                return guessNumberInternal(middle + 1, right);
            case -1:
                return guessNumberInternal(left, middle);
            default:
                return middle;
        }
    }
}

循环代替递归

那不能用递归做二分查找，能不能用循环来代替呢。
下面的代码Submit后，报错Time Limit Exceeded。擦居然超时了。
肉眼看代码检查了大半天，发现不粗破绽。

public class Solution extends GuessGame {
    public int guessNumber(int n) {
        int left = 1;
        int right = n;
        int guessNumber = 1;
        while (left <= right) {
            int middle = (left + right) / 2;
            int result = guess(middle);
            boolean find = false;
            switch (result) {
                case 1:
                    left = middle + 1;
                    break;
                case -1:
                    right = middle - 1;
                    break;
                default:
                    guessNumber = middle;
                    find = true;
                    break;
            }
            if (find) {
                break;
            }
        }
        return guessNumber;
    }
}

超时原因，int做加法时溢出了

在Java环境下，运行了上面的代码，才发现在计算middle时候，left + right这步计算int溢出了。
最后，修改代码如下，Submit，终于Accepted了。

public class Solution extends GuessGame {
    public int guessNumber(int n) {
        int left = 1;
        int right = n;
        int guessNumber = 1;
        while (left <= right) {
            int middle = left + (right - left) / 2;
            int result = guess(middle);
            boolean find = false;
            switch (result) {
                case 1:
                    left = middle + 1;
                    break;
                case -1:
                    right = middle - 1;
                    break;
                default:
                    guessNumber = middle;
                    find = true;
                    break;
            }
            if (find) {
                break;
            }
        }
        return guessNumber;
    }
}