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.

public boolean isPalindrome(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int len = s.length();
        if(len == 0)
            return true;
            
        String ignoredStr = ignore(s.toLowerCase());
        
        boolean result = checkPali(ignoredStr);
        
        return result;
    }
    
    public static String ignore(String s){
        StringBuilder sb = new StringBuilder();
        int len = s.length();
        for(int i = 0; i < len; i++){
            char c = s.charAt(i);
            if((c >= '0' && c <= '9') ||
                (c >= 'a' && c <= 'z')){
                    sb.append(c);
                }
        }
        return sb.toString();
    }
    
    public static boolean checkPali(String s){
        int len = s.length();
        boolean flag = true;
        for(int i = 0; i < (len / 2); i++){
            if(s.charAt(i) != s.charAt(len - 1 - i)){
                flag = false;
                return flag;
            }
        }
        return flag;
    }