589. N-ary Tree Preorder Traversal

https://leetcode.com/problems/n-ary-tree-preorder-traversal/

1. Use recursion:

//40 ms    11.1 MB
/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;
    Node() {}
    Node(int _val) {
        val = _val;
    }
    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
public:
    vector<int> preorder(Node* root) {
        vector<int> result;
        preorder(root, result);
        return result;
    }
private:
    void preorder(Node* root, vector<int>& result) {
        if(!root) return;
        result.push_back(root->val);
        for(auto child: root->children)
            preorder(child, result);
    }
};