Leetcode 125 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 1: two pointer move from head and tail to compare. time complexity is O(n).

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