125. 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.

public class Solution {
    public boolean isPalindrome(String s) {
        int i=0, j = s.length()-1;
        while( i < j){
            while(i<s.length() && !Character.isLetterOrDigit(s.charAt(i))) i++;
            if(i==s.length()) return true;
            while(j >=0 && !Character.isLetterOrDigit(s.charAt(j))) j--;
            if(j < 0 ) return true;

                != Character.toLowerCase(s.charAt(j))) return false;
        return true;

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