---
tags: LeetCode
---

# 9. Palindrome Number
Determine whether an integer is a palindrome. An integer is a palindrome when it reads the same backward as forward.

Example 1:
Input: 121
Output: true

Example 2:
Input: -121
Output: false
Explanation: From left to right, it reads -121. From right to left, it becomes 121-. Therefore it is not a palindrome.

Example 3:
Input: 10
Output: false
Explanation: Reads 01 from right to left. Therefore it is not a palindrome.

Follow up:
**Coud you solve it without converting the integer to a string?**

輸入範本如下

public class Solution {
        public bool IsPalindrome(int x){
        }
}

### 直覺想法
1. 因為題目要求不能把數字轉成字串. 所以只能透過餘數運算去取值. 因為要判斷是否為迴文 , 因此倒過來取值 , 若此此數字與原來的數字相同 , 則此數字是回文.

    60 ms	15.9 MB	
    You are here!
    Your runtime beats 84.28 % of csharp submissions

    public bool IsPalindrome(int x)
    {
        if (x < 0) { return false; }

        long reverse = 0, temp = x;
        while (temp != 0)
        {
            reverse = reverse * 10 + (temp % 10); // 結果可能會超過 int.MaxValue , 所以使用 long
            temp /= 10;
        }
        return reverse == x;
    }

2. 每次取出最左以及最右的數字比較 , 若不同 , 此數字非回文 , 若相同 , 則繼續比較.

56 ms	15.8 MB
You are here!
Your runtime beats 93.52 % of csharp submissions.
public bool IsPalindrome(int x)
    {
        if (x < 0) { return false; }

        int div;
        for (div = 1; (x / div) >= 10; div *= 10) ; // 找到 x 的位數 , 若 x 為個位數 , div 不應該 *10
        while (x > 0)
        {
            int left = x / div, right = x % 10;
            if (left != right) { return false; }

            x = (x % div) / 10; // 取出除了左與右外的數字.
            div /= 100;         // 因為少了兩個位數 , 所以 div 要除 100
        }
        return true;
    }

### Thank you!

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