Post order traversal of n-ary tree

import java.util.Vector;
import java.util.ArrayList;
// Java program for
// Postorder traversal of an N-ary Tree
class TreeNode
{
    public int key;
    public Vector < TreeNode > child;
    public TreeNode(int key)
    {
        // Set node key
        this.key = key;
        // Create memory of TreeNode child
        this.child = new Vector < TreeNode > ();
    }
    public void addChild(int key)
    {
        // Create a new node
        TreeNode node = new TreeNode(key);
        // Add node into child
        this.child.add(node);
    }
}
public class NAryTree
{
    public TreeNode root;

    public NAryTree()
    {
        // Set initial tree root to null
        this.root = null;
    }
   
    public void printPostOrder(TreeNode node)
    {
        if(node == null)
        {
            return;
        }
      
        for (int  i = 0; i < node.child.size() ; ++i) 
        {
            // Visit child node
            printPostOrder(node.child.get(i));
        }
        // Display node value
        System.out.print("  "+node.key);
    }
    public static void main(String[] args)
    {
        NAryTree tree = new NAryTree();
        /*
                   10
                  /   \
                 /     \
                /       \   
               8         5
              /|\      /|\ \ 
             / | \    / | \ \
            -2 1  6  7 18 3  4
              / \           /| \
             9  11         2 1  3
               /  \
              17   12   
            -----------------------
            Constructing N-Arr tree
        */
        // First element of tree
        tree.root = new TreeNode(10);
        tree.root.addChild(8);
        tree.root.addChild(5);
        // Add child node [-2,1,6] in node (8)
        tree.root.child.get(0).addChild(-2);
        tree.root.child.get(0).addChild(1);
        tree.root.child.get(0).addChild(6);
        // Add child node [9,11] in node (1)
        tree.root.child.get(0).child.get(1).addChild(9);
        tree.root.child.get(0).child.get(1).addChild(11);
        // Add child node [17  12] in node (11)
        tree.root.child.get(0).child.get(1).child.get(1).addChild(17);
        tree.root.child.get(0).child.get(1).child.get(1).addChild(12);
        // Add child node [7 18 3 4] in node (5)
        tree.root.child.get(1).addChild(7);
        tree.root.child.get(1).addChild(18);
        tree.root.child.get(1).addChild(3);
        tree.root.child.get(1).addChild(4);
        // Add child node [2,1,3] in node (4)
        tree.root.child.get(1).child.get(3).addChild(2);
        tree.root.child.get(1).child.get(3).addChild(1);
        tree.root.child.get(1).child.get(3).addChild(3);
        tree.printPostOrder(tree.root);
    }
}

Output

  -2  9  17  12  11  1  6  8  7  18  3  2  1  3  4  5  10