Check if DFS Strings Are Palindromes

class Solution {
public:
    vector<bool> findAnswer(vector<int>& parent, string s) {
        const int n = parent.size();
        vector<bool> ans(n, false);
        unordered_map<int, vector<int>> mpp;

        for(int i=0; i<n; i++){
            mpp[parent[i]].push_back(i);
        }

        for(auto& el: mpp){
            sort(el.second.begin(), el.second.end());
        }

        auto isPalindrome = [](string& str){
            int i = 0, j = str.size()-1;
            while(i < j) if(str[i++]!=str[j--]) return false;
            return true;
        };

        string DFSstring;

        function<void(int)> dfs = [&](int x){
            for(int child: mpp[x]) dfs(child);
            DFSstring += s[x];
        };

        for(int i=0; i<n; i++){
            DFSstring = "";
            dfs(i);
            ans[i] = isPalindrome(DFSstring);
        }

        return ans;
    }
};