125. Valid Palindrome java solutions

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.

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public class Solution {
    public boolean isPalindrome(String s) {
        if(s.trim() == "") return true;
        int i = 0,j = s.length()-1;
        char c1,c2;
        while(i <= j){
            c1 = s.charAt(i);
            c2 = s.charAt(j);
            if(!Character.isLetterOrDigit(c1)) i++;
            if(!Character.isLetterOrDigit(c2)) j--;
            if(Character.isLetterOrDigit(c1) && Character.isLetterOrDigit(c2)){
                if(Character.toLowerCase(c1) != Character.toLowerCase(c2)) return false;
                i++;
                j--;
            }
        }
        return true;
    }
}

主要是注意该题中 , . ! 等等不算在回文的序列中。