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:

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// 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;
}

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Output:

1 4 6 10


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