[leetcode]374. Guess Number Higher or Lower

這是leetcode第374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!

遞歸二分查找

看完題目，個人第一想法是，遞歸二分查找。因而很快就寫出以下代碼。
Submit獲得結果StackOverFlow，說明遞歸太深了。

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;
        }
    }
}

循環代替遞歸

那不能用遞歸作二分查找，能不能用循環來代替呢。
下面的代碼Submit後，報錯Time Limit Exceeded。擦竟然超時了。
肉眼看代碼檢查了大半天，發現不粗破綻。

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;
    }
}

超時緣由，int作加法時溢出了

在Java環境下，運行了上面的代碼，才發如今計算middle時候，left + right這步計算int溢出了。
最後，修改代碼以下，Submit，終於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;
    }
}