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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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