Valid String 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. Solution 1:

public class Valid_Palindrome {
    public boolean isPalindrome(String s) {
        if (s == null) {return false;}
        if (s.length() < 2) {return true;}
        char[] charArray = s.toCharArray();
        int len = s.length();
        
        int i = 0, j = len - 1;
        while (i < j) {
            char left = charArray[i];
            char right = charArray[j];
            while (i < len - 1 && !isAlpha(left) && !isNum(left)) {
                i++;
                left = charArray[i];
            }
            while (j > 0 && !isAlpha(right) && !isNum(right)) {
                j--;
                right = charArray[j];
            }
            if (i >= j) {
                break;
            }
            left = charArray[i];
            right = charArray[j];
            if (!isSame(left, right)) {
                return false;
            }
            i++;
            j--;
        }
        return true;
    }
    public boolean isAlpha(char c) {
        if (c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z') {
            return true;
        } else {
            return false;
        }
    }
    public boolean isNum(char c) {
        if (c >= '0' && c <= '9') {
            return true;
        } else {
            return false;
        }
    }
 
    public boolean isSame(char a, char b) {
        if (isNum(a) && isNum(b)) {
            return a== b;
        } else if (String.valueOf(a).equalsIgnoreCase(String.valueOf(b))) {
            return true;
        } else {
            return false;
        }
    }
 
}

Solution 2:

public class Valid_Palindrome {
    public boolean isPalindrome(String s) {
        if (s == null || s.length() == 0) {
            return true;
 }
 int front = 0;
 int end = s.length() - 1;
 while (front < end) {
  while (front < s.length() && !isValid(s.charAt(front))) {
   front++;
  }
  // for empty string
  if (front == s.length()) {
   return true;
  }
  while (end >= 0 && !isValid(s.charAt(end))) {
   end--;
  }
  // compare char
  if (Character.toLowerCase(s.charAt(front)) != Character.toLowerCase(s.charAt(end))) {
   break;
  } else {
   front++;
   end--;
  }
 }
 return end <= front;
    }
 
    private boolean isValid(char c) {
 return Character.isLetter(c) || Character.isDigit(c);
    }
        
}