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 isPalindrome(String s) {
    if (s == null) return false;
    s = s.toLowerCase();
    int n = s.length(), i = 0, j = n - 1;
    char[] chars = s.toCharArray();
    while (i < j) {
      // skip any non-alphanumeric chars
      while (i < n && !Character.isLetterOrDigit(chars[i])) { i++; }
      while (j >= 0 && !Character.isLetterOrDigit(chars[j])) { j--; }
      if (i < j && chars[i++] != chars[j--]) {
        return false;
      }
    }
    return true;
  }
}