Print alternate nodes in level order of a binary tree

Here given code implementation process.

// C program
// Print alternate nodes in level order of a binary tree
#include <stdio.h>
#include <stdlib.h>

//Node of binary tree
struct Node
{
	int data;
	struct Node *left, *right;
};
struct MyQueue
{
	int level;
	struct Node *element;
	struct MyQueue *next;
};
//Create a binary tree nodes and node fields (data,pointer) 
//And returning the reference of newly nodes
struct Node *insert(int data)
{
	//create dynamic memory to new binary tree node
	struct Node *new_node = (struct Node *) malloc(sizeof(struct Node));
	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 Node *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;
	}
}
//print all alternate nodes in binary tree in level order
void print_alternate_nodes(struct Node *root)
{
	if (root != NULL)
	{
		//make a queue pointers
		struct MyQueue *front = NULL, *tail = NULL;
		//Get first node of tree
		front = enqueue(root);
		//Start level of first node is one
		front->level = 1;
		//Set tail node to first node
		tail = front;
		// Define a tree variable
		struct Node *node = NULL;
		// Result indicator
		int status = 1;
		printf(" Alternate Nodes \n");
		// 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 (status == 1)
			{
				// When get alternate node
				printf(" %d", node->data);
				// Change selection
				status = 0;
			}
			else
			{
				// Change selection
				status = 1;
			}
			dequeue( &front);
		}
		printf("\n");
		tail = NULL;
	}
	else
	{
		printf("\nEmpty Tree\n");
	}
}
int main()
{
	struct Node *root = NULL;
	/*
	Construct Binary Tree
	-----------------------
	           10
	         /   \
	        2     3
	       /     / \
	      4     9   5
	     /  \    \    \
	    7    3    6   20
	        /  \     /
	       2    8   -8
	-----------------------
	*/
	//Add node
	root = insert(10);
	root->left = insert(2);
	root->right = insert(3);
	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(20);
	root->right->right->right->left = insert(-8);
	root->left->left->right->left = insert(2);
	root->left->left->right->right = insert(8);
	print_alternate_nodes(root);
	return 0;
}

Output

 Alternate Nodes
 10 3 9 7 6 2 -8
/* 
  Java program 
  Print alternate nodes in level order of a binary tree
*/

//Binary Tree node
class TreeNode
{
	public int data;
	public TreeNode left;
	public TreeNode right;
	public TreeNode(int data)
	{
		//set node value
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
// Queue Node
class QueueNode
{
	public TreeNode element;
	public QueueNode next;
	public int level;
	public QueueNode(TreeNode element, int level)
	{
		this.element = element;
		this.next = null;
		this.level = level;
	}
}
//Define custom queue class
class MyQueue
{
	public QueueNode front;
	public QueueNode tail;
	public MyQueue()
	{
		this.front = null;
		this.tail = null;
	}
	//Add a new node at last of queue
	public void enqueue(TreeNode element, int level)
	{
		QueueNode new_node = new QueueNode(element, level);
		if (this.front == null)
		{
			//When first node of queue
			this.front = new_node;
		}
		else
		{
			//Add node at last position
			this.tail.next = new_node;
		}
		this.tail = new_node;
	}
	//Delete first node of queue
	public void dequeue()
	{
		if (this.front != null)
		{
			if (this.tail == this.front)
			{
				this.tail = null;
				this.front = null;
			}
			else
			{
				this.front = this.front.next;
			}
		}
	}
	public boolean is_empty()
	{
		if (this.front == null)
		{
			return true;
		}
		else
		{
			return false;
		}
	}
}
class BinaryTree
{
	public TreeNode root;
	public BinaryTree()
	{
		// Set initial tree root to null
		this.root = null;
	}
	//print all alternate nodes in binary tree in level order
	public void print_alternate_nodes()
	{
		if (this.root == null)
		{
			System.out.print("\n Empty Binary Tree \n");
		}
		else
		{
			//Get top node in tree
			TreeNode node = this.root;
			int level = 1;
			//Create a Queue
			MyQueue queue = new MyQueue();
			//Add first node at the level of one
			queue.enqueue(node, level);
			// Result indicator
			boolean status = true;
			//Execute loop until the queue is not empty
			while (queue.is_empty() == false)
			{
				node = queue.front.element;
				level = queue.front.level;
				if (node.left != null)
				{
					//Add left node
					queue.enqueue(node.left, level + 1);
				}
				if (node.right != null)
				{
					//Add right node
					queue.enqueue(node.right, level + 1);
				}
				if (status == true)
				{
					status = false;
					System.out.print("  " + node.data);
				}
				else
				{
					status = true;
				}
				//remove element into queue
				queue.dequeue();
			}
		}
	}
	public static void main(String[] args)
	{
		//Object of Binary Tree
		BinaryTree tree = new BinaryTree();
		/*  
		Construct Binary Tree
		-----------------------
		           10
		         /   \
		        2     3
		       /     / \
		      4     9   5
		     /  \    \    \
		    7    3    6   20
		        /  \     /
		       2    8   -8
		-----------------------
		*/
		//Add node
		tree.root = new TreeNode(10);
		tree.root.left = new TreeNode(2);
		tree.root.right = new TreeNode(3);
		tree.root.right.right = new TreeNode(5);
		tree.root.right.left = new TreeNode(9);
		tree.root.left.left = new TreeNode(4);
		tree.root.left.left.left = new TreeNode(7);
		tree.root.left.left.right = new TreeNode(3);
		tree.root.right.left.right = new TreeNode(6);
		tree.root.right.right.right = new TreeNode(20);
		tree.root.right.right.right.left = new TreeNode(-8);
		tree.root.left.left.right.left = new TreeNode(2);
		tree.root.left.left.right.right = new TreeNode(8);
		tree.print_alternate_nodes();
	}
}

Output

  10  3  9  7  6  2  -8
//Include header file
#include <iostream>
using namespace std;

/*
  C++ program 
  Print alternate nodes in level order of a binary tree
*/

//Binary Tree node
class TreeNode
{
	public: int data;
	TreeNode *left;
	TreeNode *right;
	TreeNode(int data)
	{
		//set node value
		this->data = data;
		this->left = NULL;
		this->right = NULL;
	}
};
// Queue Node
class QueueNode
{
	public: TreeNode *element;
	QueueNode *next;
	int level;
	QueueNode(TreeNode *element, int level)
	{
		this->element = element;
		this->next = NULL;
		this->level = level;
	}
};
//Define custom queue class
class MyQueue
{
	public: QueueNode *front;
	QueueNode *tail;
	MyQueue()
	{
		this->front = NULL;
		this->tail = NULL;
	}
	//Add a new node at last of queue
	void enqueue(TreeNode *element, int level)
	{
		QueueNode *new_node = new QueueNode(element, level);
		if (this->front == NULL)
		{
			//When first node of queue
			this->front = new_node;
		}
		else
		{
			//Add node at last position
			this->tail->next = new_node;
		}
		this->tail = new_node;
	}
	//Delete first node of queue
	void dequeue()
	{
		if (this->front != NULL)
		{
			if (this->tail == this->front)
			{
				this->tail = NULL;
				this->front = NULL;
			}
			else
			{
				this->front = this->front->next;
			}
		}
	}
	bool is_empty()
	{
		if (this->front == NULL)
		{
			return true;
		}
		else
		{
			return false;
		}
	}
};
class BinaryTree
{
	public: TreeNode *root;
	BinaryTree()
	{
		// Set initial tree root to null
		this->root = NULL;
	}
	//print all alternate nodes in binary tree in level order
	void print_alternate_nodes()
	{
		if (this->root == NULL)
		{
			cout << "\n Empty Binary Tree \n";
		}
		else
		{
			//Get top node in tree
			TreeNode *node = this->root;
			int level = 1;
			//Create a Queue
			MyQueue queue = MyQueue();
			//Add first node at the level of one
			queue.enqueue(node, level);
			// Result indicator
			bool status = true;
			//Execute loop until the queue is not empty
			while (queue.is_empty() == false)
			{
				node = queue.front->element;
				level = queue.front->level;
				if (node->left != NULL)
				{
					//Add left node
					queue.enqueue(node->left, level + 1);
				}
				if (node->right != NULL)
				{
					//Add right node
					queue.enqueue(node->right, level + 1);
				}
				if (status == true)
				{
					status = false;
					cout << "  " << node->data;
				}
				else
				{
					status = true;
				}
				//remove element into queue
				queue.dequeue();
			}
		}
	}
};
int main()
{
	//Object of Binary Tree
	BinaryTree tree = BinaryTree();
	tree.root = new TreeNode(10);
	tree.root->left = new TreeNode(2);
	tree.root->right = new TreeNode(3);
	tree.root->right->right = new TreeNode(5);
	tree.root->right->left = new TreeNode(9);
	tree.root->left->left = new TreeNode(4);
	tree.root->left->left->left = new TreeNode(7);
	tree.root->left->left->right = new TreeNode(3);
	tree.root->right->left->right = new TreeNode(6);
	tree.root->right->right->right = new TreeNode(20);
	tree.root->right->right->right->left = new TreeNode(-8);
	tree.root->left->left->right->left = new TreeNode(2);
	tree.root->left->left->right->right = new TreeNode(8);
	tree.print_alternate_nodes();
	return 0;
}

Output

  10  3  9  7  6  2  -8
//Include namespace system
using System;

/* 
  C# program 
  Print alternate nodes in level order of a binary tree
*/

//Binary Tree node
class TreeNode
{
	public int data;
	public TreeNode left;
	public TreeNode right;
	public TreeNode(int data)
	{
		//set node value
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
// Queue Node
class QueueNode
{
	public TreeNode element;
	public QueueNode next;
	public int level;
	public QueueNode(TreeNode element, int level)
	{
		this.element = element;
		this.next = null;
		this.level = level;
	}
}
//Define custom queue class
class MyQueue
{
	public QueueNode front;
	public QueueNode tail;
	public MyQueue()
	{
		this.front = null;
		this.tail = null;
	}
	//Add a new node at last of queue
	public void enqueue(TreeNode element, int level)
	{
		QueueNode new_node = new QueueNode(element, level);
		if (this.front == null)
		{
			//When first node of queue
			this.front = new_node;
		}
		else
		{
			//Add node at last position
			this.tail.next = new_node;
		}
		this.tail = new_node;
	}
	//Delete first node of queue
	public void dequeue()
	{
		if (this.front != null)
		{
			if (this.tail == this.front)
			{
				this.tail = null;
				this.front = null;
			}
			else
			{
				this.front = this.front.next;
			}
		}
	}
	public Boolean is_empty()
	{
		if (this.front == null)
		{
			return true;
		}
		else
		{
			return false;
		}
	}
}
class BinaryTree
{
	public TreeNode root;
	public BinaryTree()
	{
		// Set initial tree root to null
		this.root = null;
	}
	//print all alternate nodes in binary tree in level order
	public void print_alternate_nodes()
	{
		if (this.root == null)
		{
			Console.Write("\n Empty Binary Tree \n");
		}
		else
		{
			//Get top node in tree
			TreeNode node = this.root;
			int level = 1;
			//Create a Queue
			MyQueue queue = new MyQueue();
			//Add first node at the level of one
			queue.enqueue(node, level);
			// Result indicator
			Boolean status = true;
			//Execute loop until the queue is not empty
			while (queue.is_empty() == false)
			{
				node = queue.front.element;
				level = queue.front.level;
				if (node.left != null)
				{
					//Add left node
					queue.enqueue(node.left, level + 1);
				}
				if (node.right != null)
				{
					//Add right node
					queue.enqueue(node.right, level + 1);
				}
				if (status == true)
				{
					status = false;
					Console.Write("  " + node.data);
				}
				else
				{
					status = true;
				}
				//remove element into queue
				queue.dequeue();
			}
		}
	}
	public static void Main(String[] args)
	{
		//Object of Binary Tree
		BinaryTree tree = new BinaryTree();
		tree.root = new TreeNode(10);
		tree.root.left = new TreeNode(2);
		tree.root.right = new TreeNode(3);
		tree.root.right.right = new TreeNode(5);
		tree.root.right.left = new TreeNode(9);
		tree.root.left.left = new TreeNode(4);
		tree.root.left.left.left = new TreeNode(7);
		tree.root.left.left.right = new TreeNode(3);
		tree.root.right.left.right = new TreeNode(6);
		tree.root.right.right.right = new TreeNode(20);
		tree.root.right.right.right.left = new TreeNode(-8);
		tree.root.left.left.right.left = new TreeNode(2);
		tree.root.left.left.right.right = new TreeNode(8);
		tree.print_alternate_nodes();
	}
}

Output

  10  3  9  7  6  2  -8
<?php
/* 
  Php program 
  Print alternate nodes in level order of a binary tree
*/

//Binary Tree node
class TreeNode
{
	public $data;
	public $left;
	public $right;

	function __construct($data)
	{
		//set node value
		$this->data = $data;
		$this->left = null;
		$this->right = null;
	}
}
// Queue Node
class QueueNode
{
	public $element;
	public $next;
	public $level;

	function __construct($element, $level)
	{
		$this->element = $element;
		$this->next = null;
		$this->level = $level;
	}
}
//Define custom queue class
class MyQueue
{
	public $front;
	public $tail;

	function __construct()
	{
		$this->front = null;
		$this->tail = null;
	}
	//Add a new node at last of queue
	public	function enqueue($element, $level)
	{
		$new_node = new QueueNode($element, $level);
		if ($this->front == null)
		{
			//When first node of queue
			$this->front = $new_node;
		}
		else
		{
			//Add node at last position
			$this->tail->next = $new_node;
		}
		$this->tail = $new_node;
	}
	//Delete first node of queue
	public	function dequeue()
	{
		if ($this->front != null)
		{
			if ($this->tail == $this->front)
			{
				$this->tail = null;
				$this->front = null;
			}
			else
			{
				$this->front = $this->front->next;
			}
		}
	}
	public	function is_empty()
	{
		if ($this->front == null)
		{
			return true;
		}
		else
		{
			return false;
		}
	}
}
class BinaryTree
{
	public $root;

	function __construct()
	{
		// Set initial tree root to null
		$this->root = null;
	}
	//print all alternate nodes in binary tree in level order
	public	function print_alternate_nodes()
	{
		if ($this->root == null)
		{
			echo "\n Empty Binary Tree \n";
		}
		else
		{
			//Get top node in tree
			$node = $this->root;
			$level = 1;
			//Create a Queue
			$queue = new MyQueue();
			//Add first node at the level of one
			$queue->enqueue($node, $level);
			// Result indicator
			$status = true;
			//Execute loop until the queue is not empty
			while ($queue->is_empty() == false)
			{
				$node = $queue->front->element;
				$level = $queue->front->level;
				if ($node->left != null)
				{
					//Add left node
					$queue->enqueue($node->left, $level + 1);
				}
				if ($node->right != null)
				{
					//Add right node
					$queue->enqueue($node->right, $level + 1);
				}
				if ($status == true)
				{
					$status = false;
					echo "  ". $node->data;
				}
				else
				{
					$status = true;
				}
				//remove element into queue
				$queue->dequeue();
			}
		}
	}
}

function main()
{
	//Object of Binary Tree
	$tree = new BinaryTree();
	/*  
			Construct Binary Tree
			-----------------------
			           10
			         /   \
			        2     3
			       /     / \
			      4     9   5
			     /  \    \    \
			    7    3    6   20
			        /  \     /
			       2    8   -8
			-----------------------
			*/
	//Add node
	$tree->root = new TreeNode(10);
	$tree->root->left = new TreeNode(2);
	$tree->root->right = new TreeNode(3);
	$tree->root->right->right = new TreeNode(5);
	$tree->root->right->left = new TreeNode(9);
	$tree->root->left->left = new TreeNode(4);
	$tree->root->left->left->left = new TreeNode(7);
	$tree->root->left->left->right = new TreeNode(3);
	$tree->root->right->left->right = new TreeNode(6);
	$tree->root->right->right->right = new TreeNode(20);
	$tree->root->right->right->right->left = new TreeNode(-8);
	$tree->root->left->left->right->left = new TreeNode(2);
	$tree->root->left->left->right->right = new TreeNode(8);
	$tree->print_alternate_nodes();
}
main();

Output

  10  3  9  7  6  2  -8
/* 
  Node Js program 
  Print alternate nodes in level order of a binary tree
*/

//Binary Tree node
class TreeNode
{
	constructor(data)
	{
		//set node value
		this.data = data;
		this.left = null;
		this.right = null;
	}
}
// Queue Node
class QueueNode
{
	constructor(element, level)
	{
		this.element = element;
		this.next = null;
		this.level = level;
	}
}
//Define custom queue class
class MyQueue
{
	constructor()
	{
		this.front = null;
		this.tail = null;
	}
	//Add a new node at last of queue
	enqueue(element, level)
	{
		var new_node = new QueueNode(element, level);
		if (this.front == null)
		{
			//When first node of queue
			this.front = new_node;
		}
		else
		{
			//Add node at last position
			this.tail.next = new_node;
		}
		this.tail = new_node;
	}
	//Delete first node of queue
	dequeue()
	{
		if (this.front != null)
		{
			if (this.tail == this.front)
			{
				this.tail = null;
				this.front = null;
			}
			else
			{
				this.front = this.front.next;
			}
		}
	}
	is_empty()
	{
		if (this.front == null)
		{
			return true;
		}
		else
		{
			return false;
		}
	}
}
class BinaryTree
{
	constructor()
	{
		// Set initial tree root to null
		this.root = null;
	}
	//print all alternate nodes in binary tree in level order
	print_alternate_nodes()
	{
		if (this.root == null)
		{
			process.stdout.write("\n Empty Binary Tree \n");
		}
		else
		{
			//Get top node in tree
			var node = this.root;
			var level = 1;
			//Create a Queue
			var queue = new MyQueue();
			//Add first node at the level of one
			queue.enqueue(node, level);
			// Result indicator
			var status = true;
			//Execute loop until the queue is not empty
			while (queue.is_empty() == false)
			{
				node = queue.front.element;
				level = queue.front.level;
				if (node.left != null)
				{
					//Add left node
					queue.enqueue(node.left, level + 1);
				}
				if (node.right != null)
				{
					//Add right node
					queue.enqueue(node.right, level + 1);
				}
				if (status == true)
				{
					status = false;
					process.stdout.write("  " + node.data);
				}
				else
				{
					status = true;
				}
				//remove element into queue
				queue.dequeue();
			}
		}
	}
}

function main()
{
	//Object of Binary Tree
	var tree = new BinaryTree();
	/*  
			Construct Binary Tree
			-----------------------
			           10
			         /   \
			        2     3
			       /     / \
			      4     9   5
			     /  \    \    \
			    7    3    6   20
			        /  \     /
			       2    8   -8
			-----------------------
			*/
	//Add node
	tree.root = new TreeNode(10);
	tree.root.left = new TreeNode(2);
	tree.root.right = new TreeNode(3);
	tree.root.right.right = new TreeNode(5);
	tree.root.right.left = new TreeNode(9);
	tree.root.left.left = new TreeNode(4);
	tree.root.left.left.left = new TreeNode(7);
	tree.root.left.left.right = new TreeNode(3);
	tree.root.right.left.right = new TreeNode(6);
	tree.root.right.right.right = new TreeNode(20);
	tree.root.right.right.right.left = new TreeNode(-8);
	tree.root.left.left.right.left = new TreeNode(2);
	tree.root.left.left.right.right = new TreeNode(8);
	tree.print_alternate_nodes();
}
main();

Output

  10  3  9  7  6  2  -8
#   Python 3 program 
#   Print alternate nodes in level order of a binary tree

# Binary Tree node
class TreeNode :
	
	def __init__(self, data) :
		# set node value
		self.data = data
		self.left = None
		self.right = None
	

#  Queue Node
class QueueNode :
	
	def __init__(self, element, level) :
		self.element = element
		self.next = None
		self.level = level
	

# Define custom queue class
class MyQueue :
	
	def __init__(self) :
		self.front = None
		self.tail = None
	
	# Add a new node at last of queue
	def enqueue(self, element, level) :
		new_node = QueueNode(element, level)
		if (self.front == None) :
			# When first node of queue
			self.front = new_node
		else :
			# Add node at last position
			self.tail.next = new_node
		
		self.tail = new_node
	
	# Delete first node of queue
	def dequeue(self) :
		if (self.front != None) :
			if (self.tail == self.front) :
				self.tail = None
				self.front = None
			else :
				self.front = self.front.next
			
		
	
	def is_empty(self) :
		if (self.front == None) :
			return True
		else :
			return False
		
	

class BinaryTree :
	
	def __init__(self) :
		#  Set initial tree root to null
		self.root = None
	
	# print all alternate nodes in binary tree in level order
	def print_alternate_nodes(self) :
		if (self.root == None) :
			print("\n Empty Binary Tree \n", end = "")
		else :
			# Get top node in tree
			node = self.root
			level = 1
			# Create a Queue
			queue = MyQueue()
			# Add first node at the level of one
			queue.enqueue(node, level)
			#  Result indicator
			status = True
			# Execute loop until the queue is not empty
			while (queue.is_empty() == False) :
				node = queue.front.element
				level = queue.front.level
				if (node.left != None) :
					# Add left node
					queue.enqueue(node.left, level + 1)
				
				if (node.right != None) :
					# Add right node
					queue.enqueue(node.right, level + 1)
				
				if (status == True) :
					status = False
					print("  ", node.data, end = "")
				else :
					status = True
				
				# remove element into queue
				queue.dequeue()
			
		
	

def main() :
	# Object of Binary Tree
	tree = BinaryTree()
	#   
	# 		Construct Binary Tree
	# 		-----------------------
	# 		           10
	# 		         /   \
	# 		        2     3
	# 		       /     / \
	# 		      4     9   5
	# 		     /  \    \    \
	# 		    7    3    6   20
	# 		        /  \     /
	# 		       2    8   -8
	# 		-----------------------
	# 		
	
	# Add node
	tree.root = TreeNode(10)
	tree.root.left = TreeNode(2)
	tree.root.right = TreeNode(3)
	tree.root.right.right = TreeNode(5)
	tree.root.right.left = TreeNode(9)
	tree.root.left.left = TreeNode(4)
	tree.root.left.left.left = TreeNode(7)
	tree.root.left.left.right = TreeNode(3)
	tree.root.right.left.right = TreeNode(6)
	tree.root.right.right.right = TreeNode(20)
	tree.root.right.right.right.left = TreeNode(-8)
	tree.root.left.left.right.left = TreeNode(2)
	tree.root.left.left.right.right = TreeNode(8)
	tree.print_alternate_nodes()

if __name__ == "__main__": main()

Output

   10   3   9   7   6   2   -8
#   Ruby program 
#   Print alternate nodes in level order of a binary tree

# Binary Tree node
class TreeNode  
	# Define the accessor and reader of class TreeNode  
	attr_reader :data, :left, :right
	attr_accessor :data, :left, :right
 
	
	def initialize(data) 
		# set node value
		self.data = data
		self.left = nil
		self.right = nil
	end

end

#  Queue Node
class QueueNode  
	# Define the accessor and reader of class QueueNode  
	attr_reader :element, :next, :level
	attr_accessor :element, :next, :level
 
	
	def initialize(element, level) 
		self.element = element
		self.next = nil
		self.level = level
	end

end

# Define custom queue class
class MyQueue  
	# Define the accessor and reader of class MyQueue  
	attr_reader :front, :tail
	attr_accessor :front, :tail
 
	
	def initialize() 
		self.front = nil
		self.tail = nil
	end

	# Add a new node at last of queue
	def enqueue(element, level) 
		new_node = QueueNode.new(element, level)
		if (self.front == nil) 
			# When first node of queue
			self.front = new_node
		else 
			# Add node at last position
			self.tail.next = new_node
		end

		self.tail = new_node
	end

	# Delete first node of queue
	def dequeue() 
		if (self.front != nil) 
			if (self.tail == self.front) 
				self.tail = nil
				self.front = nil
			else 
				self.front = self.front.next
			end

		end

	end

	def is_empty() 
		if (self.front == nil) 
			return true
		else 
			return false
		end

	end

end

class BinaryTree  
	# Define the accessor and reader of class BinaryTree  
	attr_reader :root
	attr_accessor :root
 
	
	def initialize() 
		#  Set initial tree root to null
		self.root = nil
	end

	# print all alternate nodes in binary tree in level order
	def print_alternate_nodes() 
		if (self.root == nil) 
			print("\n Empty Binary Tree \n")
		else 
			# Get top node in tree
			node = self.root
			level = 1
			# Create a Queue
			queue = MyQueue.new()
			# Add first node at the level of one
			queue.enqueue(node, level)
			#  Result indicator
			status = true
			# Execute loop until the queue is not empty
			while (queue.is_empty() == false) 
				node = queue.front.element
				level = queue.front.level
				if (node.left != nil) 
					# Add left node
					queue.enqueue(node.left, level + 1)
				end

				if (node.right != nil) 
					# Add right node
					queue.enqueue(node.right, level + 1)
				end

				if (status == true) 
					status = false
					print("  ", node.data)
				else 
					status = true
				end

				# remove element into queue
				queue.dequeue()
			end

		end

	end

end

def main() 
	# Object of Binary Tree
	tree = BinaryTree.new()
	#   
	# 		Construct Binary Tree
	# 		-----------------------
	# 		           10
	# 		         /   \
	# 		        2     3
	# 		       /     / \
	# 		      4     9   5
	# 		     /  \    \    \
	# 		    7    3    6   20
	# 		        /  \     /
	# 		       2    8   -8
	# 		-----------------------
	# 		
	
	# Add node
	tree.root = TreeNode.new(10)
	tree.root.left = TreeNode.new(2)
	tree.root.right = TreeNode.new(3)
	tree.root.right.right = TreeNode.new(5)
	tree.root.right.left = TreeNode.new(9)
	tree.root.left.left = TreeNode.new(4)
	tree.root.left.left.left = TreeNode.new(7)
	tree.root.left.left.right = TreeNode.new(3)
	tree.root.right.left.right = TreeNode.new(6)
	tree.root.right.right.right = TreeNode.new(20)
	tree.root.right.right.right.left = TreeNode.new(-8)
	tree.root.left.left.right.left = TreeNode.new(2)
	tree.root.left.left.right.right = TreeNode.new(8)
	tree.print_alternate_nodes()
end

main()

Output

  10  3  9  7  6  2  -8
/* 
  Scala program 
  Print alternate nodes in level order of a binary tree
*/

//Binary Tree node
class TreeNode(var data: Int,
	var left: TreeNode,
		var right: TreeNode)
{
	def this(data: Int)
	{
		this(data, null, null);
	}
}
// Queue Node
class QueueNode(var element: TreeNode,
	var next: QueueNode,
		var level: Int)
{
	def this(element: TreeNode, level: Int)
	{
		this(element, null, level);
	}
}
//Define custom queue class
class MyQueue(var front: QueueNode,
	var tail: QueueNode)
{
	def this()
	{
		this(null, null);
	}
	//Add a new node at last of queue
	def enqueue(element: TreeNode, level: Int): Unit = {
		var new_node: QueueNode = new QueueNode(element, level);
		if (this.front == null)
		{
			//When first node of queue
			this.front = new_node;
		}
		else
		{
			//Add node at last position
			this.tail.next = new_node;
		}
		this.tail = new_node;
	}
	//Delete first node of queue
	def dequeue(): Unit = {
		if (this.front != null)
		{
			if (this.tail == this.front)
			{
				this.tail = null;
				this.front = null;
			}
			else
			{
				this.front = this.front.next;
			}
		}
	}
	def is_empty(): Boolean = {
		if (this.front == null)
		{
			return true;
		}
		else
		{
			return false;
		}
	}
}
class BinaryTree(var root: TreeNode)
{
	def this()
	{
		this(null);
	}
	//print all alternate nodes in binary tree in level order
	def print_alternate_nodes(): Unit = {
		if (this.root == null)
		{
			print("\n Empty Binary Tree \n");
		}
		else
		{
			//Get top node in tree
			var node: TreeNode = this.root;
			var level: Int = 1;
			//Create a Queue
			var queue: MyQueue = new MyQueue();
			//Add first node at the level of one
			queue.enqueue(node, level);
			// Result indicator
			var status: Boolean = true;
			//Execute loop until the queue is not empty
			while (queue.is_empty() == false)
			{
				node = queue.front.element;
				level = queue.front.level;
				if (node.left != null)
				{
					//Add left node
					queue.enqueue(node.left, level + 1);
				}
				if (node.right != null)
				{
					//Add right node
					queue.enqueue(node.right, level + 1);
				}
				if (status == true)
				{
					status = false;
					print("  " + node.data);
				}
				else
				{
					status = true;
				}
				//remove element into queue
				queue.dequeue();
			}
		}
	}
}
object Main
{
	def main(args: Array[String]): Unit = {
		//Object of Binary Tree
		var tree: BinaryTree = new BinaryTree();
		/*  
				Construct Binary Tree
				-----------------------
				           10
				         /   \
				        2     3
				       /     / \
				      4     9   5
				     /  \    \    \
				    7    3    6   20
				        /  \     /
				       2    8   -8
				-----------------------
				*/
		//Add node
		tree.root = new TreeNode(10);
		tree.root.left = new TreeNode(2);
		tree.root.right = new TreeNode(3);
		tree.root.right.right = new TreeNode(5);
		tree.root.right.left = new TreeNode(9);
		tree.root.left.left = new TreeNode(4);
		tree.root.left.left.left = new TreeNode(7);
		tree.root.left.left.right = new TreeNode(3);
		tree.root.right.left.right = new TreeNode(6);
		tree.root.right.right.right = new TreeNode(20);
		tree.root.right.right.right.left = new TreeNode(-8);
		tree.root.left.left.right.left = new TreeNode(2);
		tree.root.left.left.right.right = new TreeNode(8);
		tree.print_alternate_nodes();
	}
}

Output

  10  3  9  7  6  2  -8
/* 
  Swift 4 program 
  Print alternate nodes in level order of a binary tree
*/

//Binary Tree node
class TreeNode
{
	var data: Int;
	var left: TreeNode? ;
	var right: TreeNode? ;
	init(_ data: Int)
	{
		//set node value
		self.data = data;
		self.left = nil;
		self.right = nil;
	}
}
// Queue Node
class QueueNode
{
	var element: TreeNode? ;
	var next: QueueNode? ;
	var level: Int;
	init(_ element: TreeNode? , _ level : Int)
	{
		self.element = element;
		self.next = nil;
		self.level = level;
	}
}
//Define custom queue class
class MyQueue
{
	var front: QueueNode? ;
	var tail: QueueNode? ;
	init()
	{
		self.front = nil;
		self.tail = nil;
	}
	//Add a new node at last of queue
	func enqueue(_ element: TreeNode? , _ level : Int)
	{
		let new_node: QueueNode? = QueueNode(element, level);
		if (self.front == nil)
		{
			//When first node of queue
			self.front = new_node;
		}
		else
		{
			//Add node at last position
			self.tail!.next = new_node;
		}
		self.tail = new_node;
	}
	//Delete first node of queue
	func dequeue()
	{
		if (self.front != nil)
		{
			if (self.tail === self.front)
			{
				self.tail = nil;
				self.front = nil;
			}
			else
			{
				self.front = self.front!.next;
			}
		}
	}
	func is_empty() -> Bool
	{
		if (self.front == nil)
		{
			return true;
		}
		else
		{
			return false;
		}
	}
}
class BinaryTree
{
	var root: TreeNode? ;
	init()
	{
		// Set initial tree root to null
		self.root = nil;
	}
	//print all alternate nodes in binary tree in level order
	func print_alternate_nodes()
	{
		if (self.root == nil)
		{
			print("\n Empty Binary Tree \n", terminator: "");
		}
		else
		{
			//Get top node in tree
			var node: TreeNode? = self.root;
			var level: Int = 1;
			//Create a Queue
			let queue: MyQueue = MyQueue();
			//Add first node at the level of one
			queue.enqueue(node, level);
			// Result indicator
			var status: Bool = true;
			//Execute loop until the queue is not empty
			while (queue.is_empty() == false)
			{
				node = queue.front!.element;
				level = queue.front!.level;
				if (node!.left != nil)
				{
					//Add left node
					queue.enqueue(node!.left, level + 1);
				}
				if (node!.right != nil)
				{
					//Add right node
					queue.enqueue(node!.right, level + 1);
				}
				if (status == true)
				{
					status = false;
					print("  ", node!.data, terminator: "");
				}
				else
				{
					status = true;
				}
				//remove element into queue
				queue.dequeue();
			}
		}
	}
}
func main()
{
	//Object of Binary Tree
	let tree: BinaryTree = BinaryTree();
	tree.root = TreeNode(10);
	tree.root!.left = TreeNode(2);
	tree.root!.right = TreeNode(3);
	tree.root!.right!.right = TreeNode(5);
	tree.root!.right!.left = TreeNode(9);
	tree.root!.left!.left = TreeNode(4);
	tree.root!.left!.left!.left = TreeNode(7);
	tree.root!.left!.left!.right = TreeNode(3);
	tree.root!.right!.left!.right = TreeNode(6);
	tree.root!.right!.right!.right = TreeNode(20);
	tree.root!.right!.right!.right!.left = TreeNode(-8);
	tree.root!.left!.left!.right!.left = TreeNode(2);
	tree.root!.left!.left!.right!.right = TreeNode(8);
	tree.print_alternate_nodes();
}
main();

Output

   10   3   9   7   6   2   -8


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