Replace every node with depth in n-ary tree

import java.util.Vector;
import java.util.ArrayList;
// Java program for
// Replace every node with depth in 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;
	}
	public void replaceByDepth(TreeNode node, int depth)
	{
		int i = 0;
		// Update node value by depth
		node.key = depth;
		// iterating the child of given node
		while (i < node.child.size())
		{
			// Recursively visit child node
			replaceByDepth(node.child.get(i), depth + 1);
			i++;
		}
	}
	// Display preorder traversal of binary tree
	public void printPreorder(TreeNode node)
	{
		if (node == null)
		{
			return;
		}
		int i = 0;
		TreeNode temp = null;
		// Display node value
		System.out.print("  " + node.key);
		// iterating the child of given node
		while (i < node.child.size())
		{
			temp = node.child.get(i);
			printPreorder(temp);
			i++;
		}
	}
	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
		    
		    -----------------------
		    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);
		// Before update
		System.out.print("\n Before replace \n");
		tree.printPreorder(tree.root);
		// Change node value
		tree.replaceByDepth(tree.root, 0);
		// After update
		System.out.print("\n After replace by depth \n");
		tree.printPreorder(tree.root);
	}
}

input

 Before replace
  10  8  -2  1  9  11  17  12  6  5  7  18  3  4  2  20  1  3
 After replace by depth
  0  1  2  2  3  3  4  4  2  1  2  2  2  2  3  4  3  3