Find height of n-ary tree

import java.util.Vector;
import java.util.ArrayList;
// Java program for
// Find height of n-ary tree
class TreeNode
{
    public int key;
    public Vector < TreeNode > child;
    public TreeNode(int key)
    {
        this.key = key;
        this.child = new Vector < TreeNode > ();
    }
    public void addChild(int key)
    {
        TreeNode t = new TreeNode(key);
        this.child.add(t);
    }
}
public class NAryTree
{
    public TreeNode root;
    public NAryTree()
    {
        // Set initial tree root to null
        this.root = null;
    }
    // Returns a maximum value of two numbers
    public int max(int a, int b)
    {
        if(a > b)
        {
            return a;
        }
        else
        {
            return b;
        }
    }
    public int findHeight(TreeNode node)
    {
        if (node == null)
        {
            return 0;
        }
        
        int i = 0;
        int height = 0;
        
        // iterating the child of given node
        while (i < node.child.size())
        {
            // Recursively visit child node
            height = max(findHeight(node.child.get(i)),height);
            i++;
        }
        // return new height
        return height+1;
    }
    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      20
                           |
                          -6 
            -----------------------
            Constructing N-Arr tree
        */
        // First element of tree
        tree.root = new TreeNode(10);
        tree.root.addChild(8);
        tree.root.addChild(5);
        // Add child node [-2,1,5] 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);

        // Add child node [20] in node (2)
        tree.root.child.get(1).child.get(3).child.get(0).addChild(20);
        // Add child node [-6] in node (20)
        tree.root.child.get(1).child.get(3).child.get(0)
          .child.get(0).addChild(-6);
        
        // Get tree height
        int height = tree.findHeight(tree.root);

        // Display the calculated height
        System.out.print(" Height : "+height);
    }
}

input

 Height : 6