一、题目

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.

判断一个字符串是不是回文串。

二、解题思路

字符串的回文判断问题，由于字符串可随机访问，故逐个比较首尾字符是否相等最为便利，即常见的『两根指针』技法。

两步走：

1. 找到最左边和最右边的第一个合法字符(字母或者字符)
2. 一致转换为小写进行比较

三、解题代码

public class Solution {
    public boolean isPalindrome(String s) {
        if (s == null || s.trim().isEmpty()) {
            return true;
        }

        int l = 0, r = s.length() - 1;
        while (l < r) {
            if(!Character.isLetterOrDigit(s.charAt(l))) {
                l++;
                continue;
            }
            if(!Character.isLetterOrDigit(s.charAt(r))) {
                r--;
                continue;
            }
            if (Character.toLowerCase(s.charAt(l)) == Character.toLowerCase(s.charAt(r))) {
                l++;
                r--;
            } else {
                return false;
            }
        }

        return true;
    }
}