I recenty started solving questions on leetcode and came accross this Questions where we had to remove all alphanumeric characters from a string and then check if the string is a palindrome or not.
I tried to keep O(n) time complexity still it took 54 ms which is way too long can someone please explain. Here is the code:

class Solution {
public:
    bool isPalindrome(string s) {
        int n = 0;
        transform(s.begin(),s.end(),s.begin(),::tolower);
        while(s[n] != ''){
            if(isalnum(s[n]) == 0 ){
                s.erase(s.begin()+n);
            }
            else{
            n++;
            }
        }

        string check = s;
        reverse(s.begin(),s.end());

        if(s == check){
            return true;
        }
        else{
            return false;
        }

    }
};

I was wanted to know what part of code exaclty is taking time