---
title: 'LeetCode 125. Valid Palindrome'
disqus: hackmd
---

# LeetCode 125. Valid Palindrome

## Description

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.

## Example

Input: s = "A man, a plan, a canal: Panama"
Output: true
Explanation: "amanaplanacanalpanama" is a palindrome.

## Constraints

1 <= s.length <= 2 * 10^5^
s consists only of printable ASCII characters.

## Answer

此題先抓頭尾，將非英文及數字的符號跳過。其中記得考慮全非字號的情況，若L可以一路跳到尾即為全非字號。跳完要判斷L<R看是否仍需比較，若L跟R是相同的英文 / 數字，則*L != *R和*L != *R^32(大小寫不同情況)必有一項不成立，故直接return false。剩下即為全檢查完return true。

//2022_03_12
bool isPalindrome(char * s){
    char *L = s, *R = s + strlen(s) - 1;
    while(L < R){
        while(L<R && (*L<'0') || (*L>'9' && *L<'A') || (*L>'Z' && *L<'a') || (*L>'z') ){L++;}
        while(L<R && (*R<'0') || (*R>'9' && *R<'A') || (*R>'Z' && *R<'a') || (*R>'z') ){R--;}
        if(L<R && *L != *R && *L != (*R^32)){ return false; }
        L++;
        R--;
    }
    return true;
}

## Link
https://leetcode.com/problems/valid-palindrome/
###### tags: `Leetcode`