Print extreme nodes of each level of Binary Tree in alternate order

Given a binary tree, print nodes of extreme corners of each level but in alternate order.
Example: 

For above tree, the output can be
1 2 7 8 31 
- print rightmost node of 1st level 
- print leftmost node of 2nd level 
- print rightmost node of 3rd level 
- print leftmost node of 4th level 
- print rightmost node of 5th level
OR
1 3 4 15 16 
- print leftmost node of 1st level 
- print rightmost node of 2nd level 
- print leftmost node of 3rd level 
- print rightmost node of 4th level 
- print leftmost node of 5th level

The idea is to traverse tree level by level. For each level, we count number of nodes in it and print its leftmost or the rightmost node based on value of a Boolean flag. We dequeue all nodes of current level and enqueue all nodes of next level and invert value of Boolean flag when switching levels.

Below is the implementation of above idea – 

C++

/* C++ program to print nodes of extreme corners
of each level in alternate order */
#include <bits/stdc++.h>
using namespace std;

/* A binary tree node has data, pointer to left child
and a pointer to right child */
struct Node
{
    int data;
    Node *left, *right;
};

/* Helper function that allocates a new node with the
given data and NULL left and right pointers. */
Node* newNode(int data)
{
    Node* node = new Node;
    node->data = data;
    node->right = node->left = NULL;
    return node;
}

/* Function to print nodes of extreme corners
of each level in alternate order */
void printExtremeNodes(Node* root)
{
    if (root == NULL)
        return;

    // Create a queue and enqueue left and right
    // children of root
    queue<Node*> q;
    q.push(root);

    // flag to indicate whether leftmost node or
    // the rightmost node has to be printed
    bool flag = false;
    while (!q.empty())
    {
        // nodeCount indicates number of nodes
        // at current level.
        int nodeCount = q.size();
        int n = nodeCount;

        // Dequeue all nodes of current level
        // and Enqueue all nodes of next level
        while (n--)
        {
            Node* curr = q.front();

            // Enqueue left child
            if (curr->left)
                q.push(curr->left);

            // Enqueue right child
            if (curr->right)
                q.push(curr->right);

            // Dequeue node
            q.pop();

            // if flag is true, print leftmost node
            if (flag && n == nodeCount - 1)
                cout << curr->data << " ";

            // if flag is false, print rightmost node
            if (!flag && n == 0)
                cout << curr->data << " ";
        }
        // invert flag for next level
        flag = !flag;
    }
}

/* Driver program to test above functions */
int main()
{
    // Binary Tree of Height 4
    Node* root = newNode(1);

    root->left = newNode(2);
    root->right = newNode(3);

    root->left->left  = newNode(4);
    root->left->right = newNode(5);
    root->right->right = newNode(7);

    root->left->left->left  = newNode(8);
    root->left->left->right  = newNode(9);
    root->left->right->left  = newNode(10);
    root->left->right->right  = newNode(11);
    root->right->right->left  = newNode(14);
    root->right->right->right  = newNode(15);

    root->left->left->left->left  = newNode(16);
    root->left->left->left->right  = newNode(17);
    root->right->right->right->right  = newNode(31);

    printExtremeNodes(root);

    return 0;
}

Java

// Java program to print nodes of extreme corners
//of each level in alternate order
import java.util.*;

class GFG
{

// A binary tree node has data, pointer to left child
//and a pointer to right child /
static class Node
{
    int data;
    Node left, right;
};

// Helper function that allocates a new node with the
//given data and null left and right pointers. /
static Node newNode(int data)
{
    Node node = new Node();
    node.data = data;
    node.right = node.left = null;
    return node;
}

// Function to print nodes of extreme corners
//of each level in alternate order
static void printExtremeNodes(Node root)
{
    if (root == null)
        return;

    // Create a queue and enqueue left and right
    // children of root
    Queue<Node> q = new LinkedList<Node>();
    q.add(root);

    // flag to indicate whether leftmost node or
    // the rightmost node has to be printed
    boolean flag = false;
    while (q.size()>0)
    {
        // nodeCount indicates number of nodes
        // at current level.
        int nodeCount = q.size();
        int n = nodeCount;

        // Dequeue all nodes of current level
        // and Enqueue all nodes of next level
        while (n-->0)
        {
            Node curr = q.peek();

            // Enqueue left child
            if (curr.left!=null)
                q.add(curr.left);

            // Enqueue right child
            if (curr.right!=null)
                q.add(curr.right);

            // Dequeue node
            q.remove();

            // if flag is true, print leftmost node
            if (flag && n == nodeCount - 1)
                System.out.print( curr.data + " ");

            // if flag is false, print rightmost node
            if (!flag && n == 0)
                System.out.print( curr.data + " ");
        }

        // invert flag for next level
        flag = !flag;
    }
}

// Driver code
public static void main(String args[])
{
    // Binary Tree of Height 4
    Node root = newNode(1);

    root.left = newNode(2);
    root.right = newNode(3);

    root.left.left = newNode(4);
    root.left.right = newNode(5);
    root.right.right = newNode(7);

    root.left.left.left = newNode(8);
    root.left.left.right = newNode(9);
    root.left.right.left = newNode(10);
    root.left.right.right = newNode(11);
    root.right.right.left = newNode(14);
    root.right.right.right = newNode(15);

    root.left.left.left.left = newNode(16);
    root.left.left.left.right = newNode(17);
    root.right.right.right.right = newNode(31);

    printExtremeNodes(root);
}
}

// This code is contributed by Arnab Kundu

Python

# Python program to print nodes of extreme corners
# of each level in alternate order

# Utility class to create a node
class Node:
    def __init__(self, key):
        self.data = key
        self.left = self.right = None

# Utility function to create a tree node
def newNode( data):

    temp = Node(0)
    temp.data = data
    temp.left = temp.right = None
    return temp

# Function to print nodes of extreme corners
# of each level in alternate order
def printExtremeNodes( root):

    if (root == None):
        return

    # Create a queue and enqueue left and right
    # children of root
    q = []
    q.append(root)

    # flag to indicate whether leftmost node or
    # the rightmost node has to be printed
    flag = False
    while (len(q) > 0):

        # nodeCount indicates number of nodes
        # at current level.
        nodeCount = len(q)
        n = nodeCount

        # Dequeue all nodes of current level
        # and Enqueue all nodes of next level
        while (n > 0):
            n = n - 1
            curr = q[0]

            # Enqueue left child
            if (curr.left != None):
                q.append(curr.left)

            # Enqueue right child
            if (curr.right != None):
                q.append(curr.right)

            # Dequeue node
            q.pop(0)

            # if flag is true, print leftmost node
            if (flag and n == nodeCount - 1):
                print( curr.data , end=" ")

            # if flag is false, print rightmost node
            if (not flag and n == 0):
                print( curr.data ,end= " ")

        # invert flag for next level
        flag = not flag

# Driver program to test above functions

# Binary Tree of Height 4
root = newNode(1)

root.left = newNode(2)
root.right = newNode(3)

root.left.left = newNode(4)
root.left.right = newNode(5)
root.right.right = newNode(7)

root.left.left.left = newNode(8)
root.left.left.right = newNode(9)
root.left.right.left = newNode(10)
root.left.right.right = newNode(11)
root.right.right.left = newNode(14)
root.right.right.right = newNode(15)

root.left.left.left.left = newNode(16)
root.left.left.left.right = newNode(17)
root.right.right.right.right = newNode(31)

printExtremeNodes(root)


# This code is contributed by Arnab Kundu

C#

// C# program to print nodes of extreme corners
//of each level in alternate order
using System;
using System.Collections.Generic;

class GFG
{

// A binary tree node has data, pointer to left child
//and a pointer to right child /
public class Node
{
    public int data;
    public Node left, right;
};

// Helper function that allocates a new node with the
//given data and null left and right pointers. /
static Node newNode(int data)
{
    Node node = new Node();
    node.data = data;
    node.right = node.left = null;
    return node;
}

// Function to print nodes of extreme corners
//of each level in alternate order
static void printExtremeNodes(Node root)
{
    if (root == null)
        return;

    // Create a queue and enqueue left and right
    // children of root
    Queue<Node> q = new Queue<Node>();
    q.Enqueue(root);

    // flag to indicate whether leftmost node or
    // the rightmost node has to be printed
    Boolean flag = false;
    while (q.Count > 0)
    {
        // nodeCount indicates number of nodes
        // at current level.
        int nodeCount = q.Count;
        int n = nodeCount;

        // Dequeue all nodes of current level
        // and Enqueue all nodes of next level
        while (n-->0)
        {
            Node curr = q.Peek();

            // Enqueue left child
            if (curr.left != null)
                q.Enqueue(curr.left);

            // Enqueue right child
            if (curr.right != null)
                q.Enqueue(curr.right);

            // Dequeue node
            q.Dequeue();

            // if flag is true, print leftmost node
            if (flag && n == nodeCount - 1)
                Console.Write( curr.data + " ");

            // if flag is false, print rightmost node
            if (!flag && n == 0)
                Console.Write( curr.data + " ");
        }

        // invert flag for next level
        flag = !flag;
    }
}

// Driver code
public static void Main(String []args)
{
    // Binary Tree of Height 4
    Node root = newNode(1);

    root.left = newNode(2);
    root.right = newNode(3);

    root.left.left = newNode(4);
    root.left.right = newNode(5);
    root.right.right = newNode(7);

    root.left.left.left = newNode(8);
    root.left.left.right = newNode(9);
    root.left.right.left = newNode(10);
    root.left.right.right = newNode(11);
    root.right.right.left = newNode(14);
    root.right.right.right = newNode(15);

    root.left.left.left.left = newNode(16);
    root.left.left.left.right = newNode(17);
    root.right.right.right.right = newNode(31);

    printExtremeNodes(root);
}
}

// This code is contributed by Rajput-Ji

Javascript

<script>

// JavaScript program to print nodes of extreme corners
//of each level in alternate order

// A binary tree node has data, pointer to left child
//and a pointer to right child /
class Node
{
  constructor()
  {
    this.data = 0;
    this.left = null;
    this.right = null;
  }
};

// Helper function that allocates a new node with the
//given data and null left and right pointers. /
function newNode(data)
{
    var node = new Node();
    node.data = data;
    node.right = node.left = null;
    return node;
}

// Function to print nodes of extreme corners
//of each level in alternate order
function printExtremeNodes(root)
{
    if (root == null)
        return;

    // Create a queue and enqueue left and right
    // children of root
    var q = [];
    q.push(root);

    // flag to indicate whether leftmost node or
    // the rightmost node has to be printed
    var flag = false;
    while (q.length > 0)
    {
        // nodeCount indicates number of nodes
        // at current level.
        var nodeCount = q.length;
        var n = nodeCount;

        // Dequeue all nodes of current level
        // and push all nodes of next level
        while (n-->0)
        {
            var curr = q[0];

            // push left child
            if (curr.left != null)
                q.push(curr.left);

            // push right child
            if (curr.right != null)
                q.push(curr.right);

            // Dequeue node
            q.shift();

            // if flag is true, print leftmost node
            if (flag && n == nodeCount - 1)
                document.write( curr.data + " ");

            // if flag is false, print rightmost node
            if (!flag && n == 0)
                document.write( curr.data + " ");
        }

        // invert flag for next level
        flag = !flag;
    }
}

// Driver code
// Binary Tree of Height 4
var root = newNode(1);
root.left = newNode(2);
root.right = newNode(3);
root.left.left = newNode(4);
root.left.right = newNode(5);
root.right.right = newNode(7);
root.left.left.left = newNode(8);
root.left.left.right = newNode(9);
root.left.right.left = newNode(10);
root.left.right.right = newNode(11);
root.right.right.left = newNode(14);
root.right.right.right = newNode(15);
root.left.left.left.left = newNode(16);
root.left.left.left.right = newNode(17);
root.right.right.right.right = newNode(31);
printExtremeNodes(root);


</script>

Output:

1 2 7 8 31

Time complexity of above solution is O(n) where n is total number of nodes in given binary tree.

Exercise – Print nodes of extreme corners of each level from bottom to top in alternate order.

This article is contributed by Aditya Goel. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.