LeetCode 27 Valid Palindrome

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

For example,

"A man, a plan, a canal: Panama" is a palindrome.

"race a car" is not a palindrome.

NOTE:

Have you consider that the string might be empty? This is a good question to ask during an interview.

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

分析：

判断字符串是否回文思路比较清楚，维护首位两个游标逐字符比较即可。

这个题有几个其他要求：

1，只考虑数字和字母，其他字符不考虑

2，不区分字母的大小写，即认为"A"=="a"

这实际上是判断之前对字符串进行一些预处理，应该想到正则表达式在这里可能有用武之地，把非字母数字全部删去，并将大小写统一即可。

public class Solution {
    public boolean isPalindrome(String s) {
        
        if(s == null) return false;
        //将非字母和数字的字符替换为空，即删去
        s = s.replaceAll("[^a-z0-9A-Z]","");
        if(s.length() == 0) return true;
        s = s.toLowerCase();
        
        int i = 0;
        int j = s.length()-1;
        while(i < j){
            if(s.charAt(i) != s.charAt(j))
                return false;
            i++;
            j--;
        }
        return true;
    }
}