Valid Palindrome

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.

class Solution {
public:
    bool isPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
    	int n =s.length();
		int l=0,r=n-1;
		while(l<=r)
		{
			while(l<=r&&!isalnum(s[l]))
				l++;
			while(l<=r&&!isalnum(s[r]))
				r--;
			if ( l> r)
				break;
			if (isdigit(s[l]) && s[l]!=s[r] )
				return false;
			if ( tolower(s[l])!=tolower(s[r]))
				return false;
			l++,r--;
		}
		return true;
    }
};