Valid Palindrome Leetcode

class Solution {
public:
    bool isPalindrome(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(s.size()==0)
            return true;
        vector<char> v1;
        int i;
        for(i=0;i<s.size();i++)
        {
            if(isalnum(s[i]))
                v1.push_back(tolower(s[i]));
        }
        vector<char> v2(v1);
        reverse(v2.begin(),v2.end());
        if(v1==v2)
            return true;
        return false;
    }
};