---
tags: LeetCode
---

# 125. Valid Palindrome
Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:
Input: "A man, a plan, a canal: Panama"
Output: true

Example 2:
Input: "race a car"
Output: false

輸入範本如下

public class Solution {
    public bool IsPalindrome(string s) {
        
    }
}

### 直覺想法
這題只需要判斷 '大小寫 or 數字' , 因此需要一個方式去判斷字元是否為大小寫數字. 若非大小寫數字則忽略. 使用兩個指標去指 , 看看所指的這兩個字是否相同 , 若不同 , 則此字串非迴文 , 若相同則繼續比 . 若全部比完皆相同 , 則此字串為迴文
ps : 題目的大小寫視為相同字元

72 ms	24.7 MB
You are here!
Your runtime beats 95.70 % of csharp submissions.
public bool IsPalindrome(string s)
{
    s = s.ToLower(); // 全部轉成小寫 , 就不用區分是否大小寫了
    bool ans = true;
    for (int left = 0, right = s.Length - 1; left < right; left++, right--)
    {
        while (left < right && !char.IsLetterOrDigit(s[left]))
        {
            left++;
        }
        while (left < right && !char.IsLetterOrDigit(s[right]))
        {
            right--;
        }
        if (s[left] != s[right])
        {
            return false;
        }
    }
    return ans;
}

72 ms	24.4 MB
You are here!
Your runtime beats 95.70 % of csharp submissions.
public bool IsPalindrome(string s)
{
    bool ans = true;
    for (int left = 0, right = s.Length - 1; left < right; left++, right--)
    {
        while (left < right && !IsLetterOrDigit(s[left]))
        {
            left++;
        }
        while (left < right && !IsLetterOrDigit(s[right]))
        {
            right--;
        }
        if (!IsSame(s[left], s[right]))
        {
            return false;
        }
    }
    return ans;

    bool IsLetterOrDigit(char c) => (c <= 'Z' && c >= 'A') || (c <= 'z' && c >= 'a') || (c <= '9' && c >= '0');
    bool IsSame(char a, char b) => char.ToLower(a) == char.ToLower(b);
}

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