589. N-ary Tree Preorder Traversal java

https://leetcode.com/problems/n-ary-tree-preorder-traversal/?envType=study-plan&id=programming-skills-i 

class Solution {
  public List<Integer> preorder(Node root) {
    List<Integer> ret = new ArrayList<Integer>();
    traverse(root, ret);
    return ret;
  }

  public void traverse(Node root, List<Integer> ret){
    if (root==null) return;
    ret.add(root.val);
    for (Node child: root.children){
      traverse(child, ret);
    }
  }
}