1 /*
 2 // Definition for a Node.
 3 class Node {
 4 public:
 5     int val;
 6     vector<Node*> children;
 7 
 8     Node() {}
 9 
10     Node(int _val) {
11         val = _val;
12     }
13 
14     Node(int _val, vector<Node*> _children) {
15         val = _val;
16         children = _children;
17     }
18 };
19 */
20 
21 class Solution {
22 public:
23     void preorderHelper(Node *root, vector<int> &vec) {
24         if (root == nullptr) {
25             return;
26         }
27         vec.push_back(root->val);
28         int size = root->children.size();
29         for (int i = 0; i < size; i++) {
30             preorderHelper(root->children[i], vec);
31         }
32         return;
33     }
34     vector<int> preorder(Node* root) {
35         vector<int> ans;
36         if (root == nullptr) {
37             return ans;
38         }
39         preorderHelper(root, ans);
40         return ans;
41     }
42 };

 1 class Solution {
 2 public:
 3     vector<int> preorder(Node* root) {
 4         vector<int> ans;
 5         if (root == nullptr) {
 6             return ans;
 7         }
 8         stack<Node *> s;
 9         s.push(root);
10         while (!s.empty()) {
11             Node* p = s.top();
12             s.pop();
13             ans.push_back(p->val);
14             int size = p->children.size();
15             for (int i = size - 1; i >= 0; i--) {
16                 s.push(p->children[i]);
17             }
18         }
19         return ans;
20     }
21 };