generate palindrome substrings

class Solution {
public:
    vector<vector<string>> partition(string s) {
        vector<vector<string>> ans;
        vector<string> curr;
        generate(0, s, curr, ans);

        return ans;
    }

    void generate(int idx, string& s, vector<string>& curr, vector<vector<string>>& res){
        if(idx >= s.size()) res.emplace_back(curr);

        for(int to = idx; to < s.size(); to++){
            bool valid = true;
            string tmp;
            for(int i = idx, j = to; i <= to; i++, j--){
                if(s[i] != s[j]) valid = false;
                tmp += s[i];
            }
            if(valid){
                curr.emplace_back(tmp);
                generate(to + 1, s, curr, res);
                curr.pop_back();
            }
        }
    }




};