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: 

         1
      /  |  \
     3   2   4
    / \
   5   6
// Return its preorder traversal as: [1,3,5,6,2,4].
*/
   
/*
// 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> result = new ArrayList<>();
        traversal(root, result);
        return result;
    }
    
    private void traversal(Node root, List<Integer> result) {
        if (root == null) {
            return;
        }
        result.add(root.val);
        for (Node node: root.children) {
            traversal(node, result);
        }
    }
}