Tree: Preorder Traversal challenge on hackerrank.com

[hackerrank]_Tree_Preorder_Traversal.cpp

#include <bits/stdc++.h>
#include <stack> 

using namespace std;

class Node {
	public:
		int data;
		Node *left;
		Node *right;
		
		Node(int d) 
		{
			data = d;
			left = NULL;
			right = NULL;
		}
};

class Solution {
	public:
  	Node* insert(Node* root, int data) 
	{
		if(root == NULL) 
		{
			return new Node(data);
		} 
		else 
		{
			Node* cur;
			if(data <= root->data)
			{
				cur = insert(root->left, data);
				root->left = cur;
			} 
			else 
			{
				cur = insert(root->right, data);
				root->right = cur;
			}

			return root;
		}
	}

/* you only have to complete the function given below.  
Node is defined as  

class Node {
	public:
		int data;
		Node *left;
		Node *right;
		Node(int d) {
			data = d;
			left = NULL;
			right = NULL;
		}
};

*/

	void preOrder(Node *root) 
	{
		unordered_set<Node*> metNodes;
		stack<Node*> nodeStack;
		Node* node = root;
		nodeStack.push(root);
		
		while (!nodeStack.empty())
		{
			if (metNodes.find(node) == metNodes.end()) 
			{
				metNodes.insert(node);
				cout << node->data << " ";
			}

			if (node->left != nullptr && metNodes.find(node->left) == metNodes.end())
			{
				nodeStack.push(node);
				node = node->left;
			} 
			else if (node->right != nullptr && metNodes.find(node->right) == metNodes.end()) 
			{
				nodeStack.push(node);
				node = node->right;
			} 
			else 
			{
				Node* previous = nodeStack.top();
				nodeStack.pop();
				
				node = previous;
			}
		}
	}

}; //End of Solution

int main() 
{
	Solution myTree;
	Node* root = NULL;
	
	int t;
	int data;

	std::cin >> t;

	while(t-- > 0) 
	{
		std::cin >> data;
		root = myTree.insert(root, data);
	}
  
	myTree.preOrder(root);
	return 0;
}