#define debug(x) cout<<#x<<": "<<(x)<<endl;
class Solution {
public:
    vector<int> preorder(Node* r) {
        stack<Node*>st;
        vector<int>ret;
        if(r != nullptr){
            st.push(r);
        }
        
        while(!st.empty()){
            auto t = st.top();
            st.pop();
            //debug(t->val)
            ret.push_back(t->val);
            for(auto it = t->children.rbegin();it != t->children.rend();it++){
                st.push(*it);
            }
        }
        return ret;
    }
};