589. N-ary Tree Preorder Traversal

Given an n-ary tree, return the preorder traversal of its nodes' values.

For example, given a 3-ary tree:

Return its preorder traversal as: [1,3,5,6,2,4].

Note: Recursive solution is trivial, could you do it iteratively?

先序遍历，程序如下所示：

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

    public Node() {}

    public Node(int _val,List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
    public List<Integer> preorder(Node root) {
        List<Integer> list = new ArrayList<>();
        preorder(root, list);
        return list;
    }
    public void preorder(Node root, List<Integer> list){
        if (root != null){
            list.add(root.val);
            for (Node node : root.children){
                preorder(node, list);
            }
        }
    }
}