Valid Palindrome

Problem :
Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

Note: For the purpose of this problem, we define empty string as valid palindrome.

Example 1:

Input: "A man, a plan, a canal: Panama"
Output: true
Example 2:

Input: "race a car"
Output: false
 

Constraints:

s consists only of printable ASCII characters.




Solution :C#
public class Solution {
    public bool IsPalindrome(string s) {
        if(string.IsNullOrEmpty(s))
            return true;
        if(s.Length==1)
            return true;
        
        int f=0,sec=s.Length-1;
        while(f<sec){
           while(f<sec && !Char.IsLetterOrDigit(s[f]))f++;
           while(f<sec && !Char.IsLetterOrDigit(s[sec]))sec--;
           if(f>=sec)
           {
               return true;
           }
            if(!char.ToLower(s[f]).Equals(char.ToLower(s[sec])))
            {
                    return false;
            }
            
            f++;
            sec--;
        }
        return true;
        
    }
}