string::find function is always failing on string for searching character?

I have code where I am trying to find non repeating longest string. To below function I pass string and I assign first character to temp string. Then I traverse original string character by character and see if this character is present in temp string.

To do this I am using string::find function and check std::String::npos to make sure I dont get valid index. But I always get std::string::npos. I am not sure what is wrong with find function used here

int lcs(string s) {
        std::string str;
        std::string maxstr;

        int max = -1;

        str+=s[0];

        for(int i = 1 ; i < s.length(); i++)
        {
            if(str.find(to_string(s[i])) == std::string::npos)
                str+=s[i];
            else
            {
                if(str.length() > max)
                    max = str.length();
                maxstr = str;
                str.clear();
            }
        }

        return max;
    }

int main()
{
    int i = lcs("abcabcbb");
    return 0;
}