Level order traversal in spiral form | Using Deque

Last Updated : 13 May, 2025

Given a binary tree and the task is to find the spiral order traversal of the tree and return the list containing the elements.
Spiral order Traversal mean: Starting from level 0 for root node, for all the even levels we print the node's value from right to left and for all the odd levels we print the node's value from left to right.

Examples:

Input: root = [1, 3, 2]

Output :
1
3 2
Explanation: Start with root (1), print level 0 (right to left), then level 1 (left to right).
Input : root = [10, 20, 30, 40, 60]

Output :
10
20 30
60 40
Explanation: Start with root (10), print level 0 (right to left), level 1 (left to right), and continue alternating.

Approach

We have seen recursive and iterative solutions using two stacks and an approach using one stack and one queue. In this post, a solution with one deque is discussed. The idea is to use a direction variable and decide whether to pop elements from the front or from the rear based on the value of this direction variable.

C++

// C++ program to print level order traversal
// in spiral form using one deque.
#include <bits/stdc++.h>
using namespace std;

class Node {
public:
    int data;
    Node *left, *right;
    Node(int val)
    {
        data = val;
        left = NULL;
        right = NULL;
    }
};

void spiralOrder(Node* root)
{

    deque<Node*> d;

    // Push root
    d.push_back(root);

    // Direction 0 shows print right to left
    // and for Direction 1 left to right
    int dir = 0;
    while (!d.empty()) {
        int size = d.size();
        while (size--) {
            // One whole level
            // will be print in this loop

            if (dir == 0) {
                Node* temp = d.back();
                d.pop_back();
                if (temp->right)
                    d.push_front(temp->right);
                if (temp->left)
                    d.push_front(temp->left);
                cout << temp->data << " ";
            }
            else {
                Node* temp = d.front();
                d.pop_front();
                if (temp->left)
                    d.push_back(temp->left);
                if (temp->right)
                    d.push_back(temp->right);
                cout << temp->data << " ";
            }
        }
        cout << endl;
        // Direction change
        dir = 1 - dir;
    }
}

int main()
{
    // Build the Tree
    Node* root = new Node(10);
    root->left = new Node(20);
    root->right = new Node(30);
    root->left->left = new Node(40);
    root->left->right = new Node(60);

    // Call the Function
    spiralOrder(root);

    return 0;
}

Java

// Java program to print level order traversal 
// in spiral form using one deque. 
import java.util.*; 

class GFG
{
    
static class Node 
{ 
    int data; 
    Node left, right; 
    Node(int val) 
    { 
        data = val; 
        left = null; 
        right = null; 
    } 
}; 

static void spiralOrder(Node root) 
{ 

    Deque<Node> d = new LinkedList<Node>(); 

    // Push root 
    d.addLast(root); 

    // Direction 0 shows print right to left 
    // and for Direction 1 left to right 
    int dir = 0; 
    while (d.size() > 0) 
    { 
        int size = d.size(); 
        while (size-->0)
        { 
            // One whole level 
            // will be print in this loop 

            if (dir == 0) 
            { 
                Node temp = d.peekLast(); 
                d.pollLast(); 
                if (temp.right != null) 
                    d.addFirst(temp.right); 
                if (temp.left != null) 
                    d.addFirst(temp.left); 
                System.out.print(temp.data + " "); 
            } 
            else 
            { 
                Node temp = d.peekFirst();
                d.pollFirst(); 
                if (temp.left != null) 
                    d.addLast(temp.left); 
                if (temp.right != null) 
                    d.addLast(temp.right); 
                System.out.print(temp.data + " "); 
            } 
        }
        System.out.println();
        
        // Direction change 
        dir = 1 - dir; 
    } 
}

// Driver code
public static void main(String args[]) 
{ 
    // Build the Tree 
    Node root = new Node(10); 
    root.left = new Node(20); 
    root.right = new Node(30); 
    root.left.left = new Node(40); 
    root.left.right = new Node(60); 

    // Call the Function 
    spiralOrder(root); 
} 
}

// This code is contributed by Arnab Kundu

Python3

# Python program to print level order traversal 
# in spiral form using one deque. 
class Node :
    def __init__(self,val) :
        self.data = val;
        self.left = None;
        self.right = None; 

def spiralOrder(root) :

    d = []; 

    # Push root 
    d.append(root); 

    # Direction 0 shows print right to left 
    # and for Direction 1 left to right
    direct = 0; 
    while (len(d) != 0) :
        size = len(d); 
        
        while (size) :
            size -= 1;
            
            # One whole level
            # will be print in this loop
            if (direct == 0) :
                temp = d.pop();
                
                if (temp.right) :
                    d.insert(0, temp.right);
                    
                if (temp.left) :
                    d.insert(0, temp.left);
                    
                print(temp.data, end= " ");
                
            else :
                temp = d[0];
                d.pop(0);
                
                if (temp.left) :
                    d.append(temp.left);
                    
                if (temp.right) :
                    d.append(temp.right);
                    
                    
                print(temp.data ,end= " "); 
        
        print()
        
        # Direction change 
        direct = 1 - direct; 

if __name__ == "__main__" : 

    # Build the Tree 
    root = Node(10); 
    root.left = Node(20); 
    root.right = Node(30); 
    root.left.left = Node(40); 
    root.left.right = Node(60); 

    # Call the Function 
    spiralOrder(root); 

# This code is contributed by AnkitRai01

//C# code for the above approach
using System;
using System.Collections.Generic;

class GFG
{
    class Node
    {
        public int data;
        public Node left, right;
        public Node(int val)
        {
            data = val;
            left = null;
            right = null;
        }
    }

    static void SpiralOrder(Node root)
    {
        Queue<Node> d = new Queue<Node>();

        // Push root
        d.Enqueue(root);

        // Direction 0 shows print right to left
        // and for Direction 1 left to right
        int dir = 0;
        while (d.Count > 0)
        {
            int size = d.Count;
            while (size-- > 0)
            {
                // One whole level
                // will be print in this loop

                if (dir == 1)
                {
                    Node temp = d.Peek();
                    d.Dequeue();
                    if (temp.right != null)
                        d.Enqueue(temp.right);
                    if (temp.left != null)
                        d.Enqueue(temp.left);
                    Console.Write(temp.data + " ");
                }
                else
                {
                    Node temp = d.Peek();
                    d.Dequeue();
                    if (temp.left != null)
                        d.Enqueue(temp.left);
                    if (temp.right != null)
                        d.Enqueue(temp.right);
                    Console.Write(temp.data + " ");
                }
            }
            Console.WriteLine();

            // Direction change
            dir = 1 - dir;
        }
    }

    // Driver code
  
    // Driver code
    static void Main(string[] args)
    {
        // Build the Tree
        Node root = new Node(10);
        root.left = new Node(20);
        root.right = new Node(30);
        root.left.left = new Node(40);
        root.left.right = new Node(60);

        // Call the Function
        SpiralOrder(root);
    }
}
//This code is contributed by Potta Lokesh

JavaScript

<script>
 
// JavaScript program to print level order traversal
// in spiral form using one deque.

class Node {

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

function spiralOrder(root)
{

    var d = [];

    // Push root
    d.push(root);

    // Direction 0 shows print right to left
    // and for Direction 1 left to right
    var dir = 0;
    while (d.length!=0) {
        var size = d.length;
        while (size-- >0) {
            // One whole level
            // will be print in this loop

            if (dir == 0) {
                var temp = d[d.length-1];
                d.pop();
                if (temp.right!= null)
                    d.unshift(temp.right);
                if (temp.left!= null)
                    d.unshift(temp.left);
                document.write( temp.data + " ");
            }
            else {
                var temp = d[0];
                d.shift();
                if (temp.left != null)
                    d.push(temp.left);
                if (temp.right!= null)
                    d.push(temp.right);
                document.write( temp.data + " ");
            }
        }
        document.write("<br>");
        // Direction change
        dir = 1 - dir;
    }
}

// Build the Tree
var root = new Node(10);
root.left = new Node(20);
root.right = new Node(30);
root.left.left = new Node(40);
root.left.right = new Node(60);
// Call the Function
spiralOrder(root);


</script>

Output

10 
20 30 
60 40

Time Complexity: O(N), where N is the number of Nodes
Space Complexity: O(N), where N is the number of Nodes

Level order traversal in spiral form | Using Deque

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