//DFS recursive solution
//Time: O(n), 3ms
//Space: O(h)
class Solution {
    public List<Integer> preorder(Node root) {
        List<Integer> ans = new ArrayList<>();
        DFS(root, ans);
        return ans;
    }

    private void DFS(Node root, List<Integer> ans) {
        if(root == null) return;
        ans.add(root.val);
        for(Node child : root.children) {
            DFS(child, ans);
        }
    }
}
/*
// 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;
    }
};
*/