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Print alternate nodes in level order of a binary tree

In this problem, we are given a binary tree, and we are required to print all the alternate nodes in level order. In level order traversal, we visit the nodes level by level, from left to right.

Example

Consider the following binary tree:


       10
     /   \
    2     3
   /     / \
  4     9   5
 /  \    \    \
7    3    6   20
    /  \     /
   2    8   -8

The alternate nodes in level order are: 10, 3, 9, 7, 6, 2, -8

Idea to Solve the Problem

To print the alternate nodes in level order, we can perform a level order traversal of the binary tree using a custom queue data structure. For each level, we can print the alternate nodes based on a flag. We alternate the flag value for each level, so that alternate nodes are printed.

Code Solution

// 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

Algorithm

  1. Define the TreeNode class to represent a node in the binary tree. Each node will have a data value and pointers to its left and right children.
  2. Define the QueueNode class to represent a node in the custom queue. Each node will contain a pointer to a binary tree node, its level, and a link to the next node in the queue.
  3. Define the MyQueue class to implement the custom queue. It will have methods to enqueue and dequeue nodes, as well as a method to check if the queue is empty.
  4. Define the BinaryTree class to represent the binary tree. It will have a method print_alternate_nodes() to print all the alternate nodes in level order.
  5. Perform a level order traversal of the binary tree using a queue. For each level, maintain a flag to decide whether to print the alternate nodes or not.

Time Complexity

  • The time complexity of performing a level order traversal of the binary tree to print alternate nodes is O(N), where N is the number of nodes in the tree.
  • Therefore, the overall time complexity of the algorithm is O(N), where N is the number of nodes in the binary tree.




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