原题链接：https://leetcode-cn.com/problems/n-ary-tree-preorder-traversal/

vector<int> preorder(Node* root) {
    vector<int> res;
    stack<Node*> sk;
    if (root) sk.push(root);
    while(sk.size()) {
        Node* node = sk.top();
        sk.pop();
        res.push_back(node->val);
        int n = node->children.size();
        for (int i = n - 1; i >= 0; --i) {
            sk.push(node->children[i]);
        }
    }
    return res;
}