Print even positioned nodes of even levels in level order of the given binary tree

Given a binary tree, print even positioned nodes of even level in level order traversal. The root is considered at level 0, and the left most node of any level is considered as a node at position 0. 
Examples:

Input:
         1
       /   \
      2     3
    /   \     \
   4     5      6
        /  \
       7    8
      /      \
     9        10

Output: 1 4 6 9

Input:
      2
    /   \
   4     15
  /     /
 45   17

Output: 2 45

Approach: To print nodes level by level, use level order traversal. The idea is based on Print level order traversal line by line. For that, traverse nodes level by level and switch even level flag after every level. Similarly, mark 1st node in every level as even position and switch it after each time the next node is processed.
Below is the implementation of the above approach: 

C++

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

struct Node {
    int data;
    Node *left, *right;
};

// Iterative method to do level order
// traversal line by line
void printEvenLevelEvenNodes(Node* root)
{
    // Base Case
    if (root == NULL)
        return;

    // Create an empty queue for level
    // order traversal
    queue<Node*> q;

    // Enqueue root and initialize level as even
    q.push(root);
    bool evenLevel = true;

    while (1) {

        // nodeCount (queue size) indicates
        // number of nodes in the current level
        int nodeCount = q.size();
        if (nodeCount == 0)
            break;

        // Mark 1st node as even positioned
        bool evenNodePosition = true;

        // Dequeue all the nodes of current level
        // and Enqueue all the nodes of next level
        while (nodeCount > 0) {
            Node* node = q.front();

            // Print only even positioned
            // nodes of even levels
            if (evenLevel && evenNodePosition)
                cout << node->data << " ";
            q.pop();
            if (node->left != NULL)
                q.push(node->left);
            if (node->right != NULL)
                q.push(node->right);
            nodeCount--;

            // Switch the even position flag
            evenNodePosition = !evenNodePosition;
        }

        // Switch the even level flag
        evenLevel = !evenLevel;
    }
}

// Utility method to create a node
struct Node* newNode(int data)
{
    struct Node* node = new Node;
    node->data = data;
    node->left = node->right = NULL;
    return (node);
}

// Driver code
int main()
{
    struct Node* root = newNode(1);
    root->left = newNode(2);
    root->right = newNode(3);
    root->left->left = newNode(4);
    root->left->right = newNode(5);
    root->right->left = newNode(6);
    root->right->right = newNode(7);
    root->left->right->left = newNode(8);
    root->left->right->right = newNode(9);
    root->left->right->right->right = newNode(10);

    printEvenLevelEvenNodes(root);

    return 0;
}

Java

// Java implementation of the approach
import java.util.*;

class GFG
{

static class Node
{
    int data;
    Node left, right;
};

// Iterative method to do level order
// traversal line by line
static void printEvenLevelEvenNodes(Node root)
{
    // Base Case
    if (root == null)
        return;

    // Create an empty queue for level
    // order traversal
    Queue<Node> q = new LinkedList<Node>();

    // Enqueue root and initialize level as even
    q.add(root);
    boolean evenLevel = true;

    while (true)
    {

        // nodeCount (queue size) indicates
        // number of nodes in the current level
        int nodeCount = q.size();
        if (nodeCount == 0)
            break;

        // Mark 1st node as even positioned
        boolean evenNodePosition = true;

        // Dequeue all the nodes of current level
        // and Enqueue all the nodes of next level
        while (nodeCount > 0)
        {
            Node node = q.peek();

            // Print only even positioned
            // nodes of even levels
            if (evenLevel && evenNodePosition)
                System.out.print(node.data + " ");
            q.remove();
            if (node.left != null)
                q.add(node.left);
            if (node.right != null)
                q.add(node.right);
            nodeCount--;

            // Switch the even position flag
            evenNodePosition = !evenNodePosition;
        }

        // Switch the even level flag
        evenLevel = !evenLevel;
    }
}

// Utility method to create a node
static Node newNode(int data)
{
    Node node = new Node();
    node.data = data;
    node.left = node.right = null;
    return (node);
}

// Driver code
public static void main(String[] args)
{
    Node root = newNode(1);
    root.left = newNode(2);
    root.right = newNode(3);
    root.left.left = newNode(4);
    root.left.right = newNode(5);
    root.right.left = newNode(6);
    root.right.right = newNode(7);
    root.left.right.left = newNode(8);
    root.left.right.right = newNode(9);
    root.left.right.right.right = newNode(10);

    printEvenLevelEvenNodes(root);
}
}

// This code is contributed by 29AjayKumar

Python3

# Python3 implementation of the approach

# Utility method to create a node
class newNode:

    # Construct to create a new node
    def __init__(self, key):
        self.data = key
        self.left = None
        self.right = None

# Iterative method to do level order
# traversal line by line
def printEvenLevelEvenNodes(root):

    # Base Case
    if (root == None):
        return

    # Create an empty queue for level
    # order traversal
    q = []

    # Enqueue root and initialize
    # level as even
    q.append(root)
    evenLevel = True

    while (1):

        # nodeCount (queue size) indicates
        # number of nodes in the current level
        nodeCount = len(q)
        if (nodeCount == 0):
            break

        # Mark 1st node as even positioned
        evenNodePosition = True

        # Dequeue all the nodes of current level
        # and Enqueue all the nodes of next level
        while (nodeCount > 0):
            node = q[0]

            # Print only even positioned
            # nodes of even levels
            if (evenLevel and evenNodePosition):
                print(node.data, end = " ")
            q.pop(0)
            if (node.left != None):
                q.append(node.left)
            if (node.right != None):
                q.append(node.right)
            nodeCount -= 1

            # Switch the even position flag
            evenNodePosition = not evenNodePosition

        # Switch the even level flag
        evenLevel = not evenLevel

# Driver code
if __name__ == '__main__':

    root = newNode(1)
    root.left = newNode(2)
    root.right = newNode(3)
    root.left.left = newNode(4)
    root.left.right = newNode(5)
    root.right.left = newNode(6)
    root.right.right = newNode(7)
    root.left.right.left = newNode(8)
    root.left.right.right = newNode(9)
    root.left.right.right.right = newNode(10)

    printEvenLevelEvenNodes(root)

# This code is contributed by SHUBHAMSINGH10

C#

// C# implementation of the approach
using System;
using System.Collections.Generic;

class GFG
{
public class Node
{
    public int data;
    public Node left, right;
};

// Iterative method to do level order
// traversal line by line
static void printEvenLevelEvenNodes(Node root)
{
    // Base Case
    if (root == null)
        return;

    // Create an empty queue for level
    // order traversal
    Queue<Node> q = new Queue<Node> ();

    // Enqueue root and initialize level as even
    q.Enqueue(root);
    bool evenLevel = true;

    while (true)
    {

        // nodeCount (queue size) indicates
        // number of nodes in the current level
        int nodeCount = q.Count;
        if (nodeCount == 0)
            break;

        // Mark 1st node as even positioned
        bool evenNodePosition = true;

        // Dequeue all the nodes of current level
        // and Enqueue all the nodes of next level
        while (nodeCount > 0)
        {
            Node node = q.Peek();

            // Print only even positioned
            // nodes of even levels
            if (evenLevel && evenNodePosition)
                Console.Write(node.data + " ");
            q.Dequeue();
            if (node.left != null)
                q.Enqueue(node.left);
            if (node.right != null)
                q.Enqueue(node.right);
            nodeCount--;

            // Switch the even position flag
            evenNodePosition = !evenNodePosition;
        }

        // Switch the even level flag
        evenLevel = !evenLevel;
    }
}

// Utility method to create a node
static Node newNode(int data)
{
    Node node = new Node();
    node.data = data;
    node.left = node.right = null;
    return (node);
}

// Driver code
public static void Main(String[] args)
{
    Node root = newNode(1);
    root.left = newNode(2);
    root.right = newNode(3);
    root.left.left = newNode(4);
    root.left.right = newNode(5);
    root.right.left = newNode(6);
    root.right.right = newNode(7);
    root.left.right.left = newNode(8);
    root.left.right.right = newNode(9);
    root.left.right.right.right = newNode(10);

    printEvenLevelEvenNodes(root);
}
}

// This code is contributed by PrinciRaj1992

Javascript

<script>

    // JavaScript implementation of the approach

    class Node
    {
        constructor(data) {
           this.left = null;
           this.right = null;
           this.data = data;
        }
    }

    // Iterative method to do level order
    // traversal line by line
    function printEvenLevelEvenNodes(root)
    {
        // Base Case
        if (root == null)
            return;

        // Create an empty queue for level
        // order traversal
        let q = [];

        // Enqueue root and initialize level as even
        q.push(root);
        let evenLevel = true;

        while (true)
        {

            // nodeCount (queue size) indicates
            // number of nodes in the current level
            let nodeCount = q.length;
            if (nodeCount == 0)
                break;

            // Mark 1st node as even positioned
            let evenNodePosition = true;

            // Dequeue all the nodes of current level
            // and Enqueue all the nodes of next level
            while (nodeCount > 0)
            {
                let node = q[0];

                // Print only even positioned
                // nodes of even levels
                if (evenLevel && evenNodePosition)
                    document.write(node.data + " ");
                q.shift();
                if (node.left != null)
                    q.push(node.left);
                if (node.right != null)
                    q.push(node.right);
                nodeCount--;

                // Switch the even position flag
                evenNodePosition = !evenNodePosition;
            }

            // Switch the even level flag
            evenLevel = !evenLevel;
        }
    }

    // Utility method to create a node
    function newNode(data)
    {
        let node = new Node(data);
        return (node);
    }

    let root = newNode(1);
    root.left = newNode(2);
    root.right = newNode(3);
    root.left.left = newNode(4);
    root.left.right = newNode(5);
    root.right.left = newNode(6);
    root.right.right = newNode(7);
    root.left.right.left = newNode(8);
    root.left.right.right = newNode(9);
    root.left.right.right.right = newNode(10);

    printEvenLevelEvenNodes(root);

</script>

Output:

1 4 6 10

Time Complexity: O(N) where n is the number of nodes in the binary tree.
Auxiliary Space: O(N) where n is the number of nodes in the binary tree.