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## Problem

Given the root of an n-ary tree, return the postorder 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)

Example 1:

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


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: [2,6,14,11,7,3,12,8,4,13,9,10,5,1]


Constraints:

• The number of nodes in the tree is in the range $[0, 10^4]$.
• $0 <= Node.val <= 10^4$
• The height of the n-ary tree is less than or equal to 1000.

Follow up: Recursive solution is trivial, could you do it iteratively?

## Code

/*
// Definition for a Node.
class Node {
public int val;
public List<Node> children;

public Node() {}

public Node(int _val) {
val = _val;
}

public Node(int _val, List<Node> _children) {
val = _val;
children = _children;
}
};
*/

class Solution {
public List<Integer> postorder(Node root) {
List<Integer> list = new ArrayList<>();
helper(root, list);
return list;
}

private void helper(Node root, List<Integer> list) {
if(root == null) return;

for(Node node : root.children) {
helper(node, list);
}

}
}

/*
// Definition for a Node.
class Node {
public int val;
public List<Node> children;

public Node() {}

public Node(int _val) {
val = _val;
}

public Node(int _val, List<Node> _children) {
val = _val;
children = _children;
}
};
*/

class Solution {
public List<Integer> postorder(Node root) {
List<Integer> list = new ArrayList<>();
if (root == null) return list;

Stack<Node> stack = new Stack<>();

stack.push(root);

while(!stack.isEmpty()) {
root = stack.pop();