LeetCode - 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.

Solution:

public class Solution {
    public boolean isValid(char c){
        if(c >= 'a' && c <= 'z') return true;
        if(c >= '0' && c <= '9') return true;
        return false;
    }
    public boolean isPalindrome(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(s.equals("")) return true;
        s = s.toLowerCase();
        int len = s.length();
        int left = 0, right = len - 1;
        while(left < right){
            while(!isValid(s.charAt(left))){
                left++;
                if(left >= right) return true;
            } 
            while(!isValid(s.charAt(right))){
                right--;
                if(left >= right) return true;
            }
            if(s.charAt(left) != s.charAt(right)) return false;
            left++;
            right--;
        }
        return true;
    }
}

 

More strictly, 

public class Solution {
    public boolean isPalindrome(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(s.equals("")) return true;
        int len = s.length();
        for(int i = 0; i <= len / 2; i++) {
            if(s.charAt(i) != s.charAt(len - 1 - i))
                return false;
        }
        return true;
    }
}