Print the boundary nodes in each level of the binary tree using queue

// C program
// Print the boundary nodes in each level of the binary tree using queue
#include <stdio.h>
#include <stdlib.h>

// Node of binary tree
struct TreeNode
{
	int data;
	struct TreeNode *left, *right;
};
struct MyQueue
{
	int level;
	struct TreeNode *element;
	struct MyQueue *next;
};
// Create a binary tree nodes and node fields (data,pointer) 
// And returning the reference of newly nodes
struct TreeNode *insert(int data)
{
	// Create dynamic memory of tree node
	struct TreeNode *new_node = 
      (struct TreeNode *) malloc(sizeof(struct TreeNode));
	if (new_node != NULL)
	{
		// Set node value
		new_node->data = data;
		new_node->left = NULL;
		new_node->right = NULL;
	}
	else
	{
		printf("Memory Overflow\n");
	}
	// return reference
	return new_node;
}
// Create a queue node and returns this node
struct MyQueue *enqueue(struct TreeNode *tree_node)
{
	// Make a new Queue node
	struct MyQueue *new_node = 
      (struct MyQueue *) malloc(sizeof(struct MyQueue));
	if (new_node != NULL)
	{
		// Set node values
		new_node->element = tree_node;
		new_node->next = NULL;
	}
	else
	{
		printf("Memory Overflow\n");
	}
	return new_node;
}
// Remove a queue elements
void dequeue(struct MyQueue **front)
{
	if ( *front != NULL)
	{
		struct MyQueue *remove = *front;
		//Visit to next node
		*front = remove->next;
		remove->element = NULL;
		remove->next = NULL;
		//free node
		free(remove);
		remove = NULL;
	}
}
void findBoundaryLevelNode(struct TreeNode *root)
{
	if (root != NULL)
	{
		// make a queue pointers
		struct MyQueue *front = NULL, *tail = NULL;
		// Get first node of tree
		front = enqueue(root);
		// Assume first node level is zero
		// Start level of first node is zero
		front->level = 0;
		//Set tail node to first node
		tail = front;
		// Define tree variables
		struct TreeNode *node = NULL;
		struct TreeNode *first = NULL;
		struct TreeNode *last = NULL;
		int level = -1;
		// Traversal tree elements in level order
		while (front != NULL)
		{
			// Tree node
			node = front->element;
			if (node->left != NULL)
			{
				// Add new left child node
				tail->next = enqueue(node->left);
				tail->next->level = front->level + 1;
				tail = tail->next;
			}
			if (node->right != NULL)
			{
				// Add new right child node
				tail->next = enqueue(node->right);
				tail->next->level = front->level + 1;
				tail = tail->next;
			}
			if (front->level != level)
			{
				level = front->level;
				if (last != NULL && last != first)
				{
					// Print the last node in the above level
					printf("  %d", last->data);
				}
				first = node;
				// Print first node in current level
				printf("\n %d", node->data);
			}
			last = node;
			// Remove queue node
			dequeue( & front);
		}
		if (last != NULL && last != first)
		{
			// Print the last node in the above level
			printf("  %d\n", last->data);
		}
		tail = NULL;
	}
	else
	{
		printf("\nEmpty Tree\n");
	}
}
int main()
{
	struct TreeNode *root = NULL;
	/* 
	        Binary Tree
	    -----------------------
	          10             
	         /   \
	        2    13         
	       /     / \                      -
	      4     9   5         
	     /  \    \   \                  -
	    7    3    6   11     
	        /  \     /  \                -    -
	       2    8   -3  12   
	           / \    
	         -1   9         
	                                        -
	    ------------------------
	*/
	//Add node
	root = insert(10);
	root->left = insert(2);
	root->right = insert(13);
	root->right->right = insert(5);
	root->right->left = insert(9);
	root->left->left = insert(4);
	root->left->left->left = insert(7);
	root->left->left->right = insert(3);
	root->right->left->right = insert(6);
	root->right->right->right = insert(11);
	root->right->right->right->right = insert(12);
	root->right->right->right->left = insert(-3);
	root->left->left->right->left = insert(2);
	root->left->left->right->right = insert(8);
	root->left->left->right->right->left = insert(-1);
	root->left->left->right->right->right = insert(9);
	/* 
	 Boundary Nodes 
	------------------------------
	           ↓
	          10            
	         /   \
	      → 2     13 ←      
	       /     / \                      -
	   →  4     9   5 ←   
	     /  \    \    \                  -
	 →  7    3    6   11 ←   
	        /  \     /  \                -    -
	    →  2    8   -3  12 ←
	           / \    
	        → -1  9 ←       
	                                    -
	---------------------------------
	*/
	findBoundaryLevelNode(root);
	return 0;
}

Output

 10
 2  13
 4  5
 7  11
 2  12
 -1  9