Given a binary tree, how do we print the left view of it?

LeftViewBinaryTree.cpp

#include <iostream>
#include <queue>

using namespace std;

//Binary tree node type
struct TreeNode
{
	int val;
	TreeNode *left, *right;
	TreeNode(int v):val(v),left(NULL), right(NULL)
	{
	}
};

//Binary search tree insert procedure for creating binary tree to test
TreeNode* insert(TreeNode * root, int data)
{
	if( root == NULL )
	{
		root = new TreeNode(data);
		return root;
	}
	if( data < root->val )
		root->left = insert( root->left, data );
	else
		root->right = insert( root->right, data );
	return root;
}

//Use level order traversing to find out the first node in each level
//nodes at a particular level in the queue are separated by NULL node
void printLeftView(TreeNode *root)
{
	if( root == NULL )
		return;
	
	queue<TreeNode*> nodeQ;

	nodeQ.push(root);
	nodeQ.push(NULL);
	
	cout << root->val << " ";

	while( !nodeQ.empty() )
	{
		TreeNode *temp = nodeQ.front();
		nodeQ.pop();
		if( temp == NULL )
		{			
			if( !nodeQ.empty() )
			{
				temp = nodeQ.front();
				cout << temp->val << " ";
				nodeQ.pop();
				nodeQ.push(NULL);
			}
		}
		if( temp != NULL )
		{
			if( temp->left != NULL )
				nodeQ.push(temp->left);
			if( temp->right != NULL )
				nodeQ.push(temp->right);		
		}
	}
	cout << endl;
}

void printLeftViewRecursive(TreeNode *root, int &maxLevel, int currentLevel)
{
	if( root == NULL )
		return;

	if( maxLevel < currentLevel )
	{
		cout << root->val << " ";
		maxLevel = currentLevel;
	}
	printLeftViewRecursive( root->left, maxLevel, currentLevel+1 );
	printLeftViewRecursive( root->right, maxLevel, currentLevel+1 );
}

void leftViewRecursive(TreeNode *root)
{
	int maxLevel = -1;
	printLeftViewRecursive(root, maxLevel, 0);
}

void Test1()
{
	TreeNode *root = NULL;
	root = insert(root, 2);
	root = insert(root, 1);
	root = insert(root, 4);
	root = insert(root, 3);
	root = insert(root, 5);
	printLeftView(root);
	leftViewRecursive(root);
	cout << endl;
}

void Test2()
{
	TreeNode *root = NULL;
	root = insert(root, 4);
	root = insert(root, 2);
	root = insert(root, 5);
	root = insert(root, 1);
	root = insert(root, 3);
	root = insert(root, 6);
	printLeftView(root);
	leftViewRecursive(root);
	cout << endl;
}

int main()
{
	Test1();
	Test2();
	return 0;
}