leetcode589+多叉树的前序遍历，递归

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

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;

    Node() {}

    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
public:
    vector<int> res;
    
    vector<int> preorder(Node* root) {
        if(root!=NULL){
            res.push_back(root->val);
            for(auto n: root->children)
                preorder(n);
        }
        return res;
    }
};