589. N-ary Tree Preorder Traversal

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

Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value (See examples).

Follow up:

Recursive solution is trivial, could you do it iteratively?

Example 1:

Input: root = [1,null,3,2,4,null,5,6]
Output: [1,3,5,6,2,4]

Example 2:

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [1,2,3,6,7,11,14,4,8,12,5,9,13,10]

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

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

 stack是先入后出，所以可以使用，queue就不行