leetcode：Valid Palindrome 【Java】

一、问题描述

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.
二、问题分析

用Character.isLetterOrDigit方法（确定指定字符是否为字母或数字）排除格式字符的干扰。

三、算法代码

public class Solution {
    public boolean isPalindrome(String s) {
        if (s == null){
            return false;
        }
        //空字符串返回为true
        if (s.length() == 0)
            return true;
            
        int head = 0, tail = s.length() - 1;
        s = s.toLowerCase();
        while (head <= tail) {
            while (head < tail && !Character.isLetterOrDigit(s.charAt(head)))
                head++;
    
            while (head < high && !Character.isLetterOrDigit(s.charAt(tail)))
                tail--;
    
            if (s.charAt(head) != s.charAt(tail))
                return false;
            head++;
            tail--;
        }
        return true;
    }
}