class Solution {
    public List<Integer> preorder(Node root) {
        List<Integer> list = new ArrayList<>();
        if(root == null){
            return list;
        }
        list.add(root.val);
        List<Node> children = root.children;
        if(children!=null){
            for(Node child : children){
                list.addAll(preorder(child));
            }
        }
        return list;
    }
}

class Solution {
    public List<Integer> preorder(Node root) {
        List<Integer> list = new ArrayList<>();
        if(root == null){
            return list;
        }
        LinkedList<Node> stack = new LinkedList<>();
        stack.push(root);
        while(!stack.isEmpty()){
            root = stack.pop();
            list.add(root.val);
            List<Node> children = root.children;
            if(children!=null){
                for(int i = children.size() - 1;i >= 0;i--){
                    stack.push(children.get(i));
                }
            }
        }
        return list;
    }
}