I recently started leetcode and ive been getting better. I attempted a problem and i got through all 479/485 test cases. But my code is struggling with a large set of data.

I tried completing the code on my own, and it works good on small sets. Can somebody explain to me how I can make it more efficient with large sets of data, and what makes mine struggle.

Here is the Question: A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.

Here is my code:

class Solution {
public:
    bool isPalindrome(string s) {
        string noSpaceString = "";
        for(int i = 0; i < s.length(); i++)
        {
            s[i] = tolower(s[i]);

            if((int(s[i]) >= 97 && int(s[i]) <= 122) || (int(s[i]) >= 48 && int(s[i] <= 57)))
            {
                noSpaceString = noSpaceString + s[i]; 
            }
        }

        string reverseStr = noSpaceString; 
        reverse(reverseStr.begin(), reverseStr.end());

        if(noSpaceString == reverseStr)
             return true;
        else 
             return false;
    }
};