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Reverse Level Order Traversal

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We have discussed the level-order traversal of a tree in the previous post. The idea is to print the last level first, then the second last level, and so on. Like Level order traversal, every level is printed from left to right.
 

Example Tree

The reverse Level order traversal of the above tree is “4 5 2 3 1”. 
Both methods for normal level order traversal can be easily modified to do reverse level order traversal.

Recommended Practice

METHOD 1 (Recursive function to print a given level) 
We can easily modify method 1 of the normal level order traversal. In method 1, we have a method printGivenLevel() which prints a given level number. The only thing we need to change is, instead of calling printGivenLevel() from the first level to the last level, we call it from the last level to the first level. 

C++




// A recursive C++ program to print
// REVERSE level order traversal
#include <bits/stdc++.h>
using namespace std;
 
/* A binary tree node has data,
pointer to left and right child */
class node
{
    public:
    int data;
    node* left;
    node* right;
};
 
/*Function prototypes*/
void printGivenLevel(node* root, int level);
int height(node* node);
node* newNode(int data);
 
/* Function to print REVERSE
level order traversal a tree*/
void reverseLevelOrder(node* root)
{
    int h = height(root);
    int i;
    for (i=h; i>=1; i--) //THE ONLY LINE DIFFERENT FROM NORMAL LEVEL ORDER
        printGivenLevel(root, i);
}
 
/* Print nodes at a given level */
void printGivenLevel(node* root, int level)
{
    if (root == NULL)
        return;
    if (level == 1)
        cout << root->data << " ";
    else if (level > 1)
    {
        printGivenLevel(root->left, level - 1);
        printGivenLevel(root->right, level - 1);
    }
}
 
/* Compute the "height" of a tree -- the number of
    nodes along the longest path from the root node
    down to the farthest leaf node.*/
int height(node* node)
{
    if (node == NULL)
        return 0;
    else
    {
        /* compute the height of each subtree */
        int lheight = height(node->left);
        int rheight = height(node->right);
 
        /* use the larger one */
        if (lheight > rheight)
            return(lheight + 1);
        else return(rheight + 1);
    }
}
 
/* 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->left = NULL;
    Node->right = NULL;
 
    return(Node);
}
 
/* Driver code*/
int main()
{
    node *root = newNode(1);
    root->left = newNode(2);
    root->right = newNode(3);
    root->left->left = newNode(4);
    root->left->right = newNode(5);
 
    cout << "Level Order traversal of binary tree is \n";
    reverseLevelOrder(root);
 
    return 0;
 
}
 
// This code is contributed by rathbhupendra


C




// A recursive C program to print REVERSE level order traversal
#include <stdio.h>
#include <stdlib.h>
 
/* A binary tree node has data, pointer to left and right child */
struct node
{
    int data;
    struct node* left;
    struct node* right;
};
 
/*Function prototypes*/
void printGivenLevel(struct node* root, int level);
int height(struct node* node);
struct node* newNode(int data);
 
/* Function to print REVERSE level order traversal a tree*/
void reverseLevelOrder(struct node* root)
{
    int h = height(root);
    int i;
    for (i=h; i>=1; i--) //THE ONLY LINE DIFFERENT FROM NORMAL LEVEL ORDER
        printGivenLevel(root, i);
}
 
/* Print nodes at a given level */
void printGivenLevel(struct node* root, int level)
{
    if (root == NULL)
        return;
    if (level == 1)
        printf("%d ", root->data);
    else if (level > 1)
    {
        printGivenLevel(root->left, level-1);
        printGivenLevel(root->right, level-1);
    }
}
 
/* Compute the "height" of a tree -- the number of
    nodes along the longest path from the root node
    down to the farthest leaf node.*/
int height(struct node* node)
{
    if (node==NULL)
        return 0;
    else
    {
        /* compute the height of each subtree */
        int lheight = height(node->left);
        int rheight = height(node->right);
 
        /* use the larger one */
        if (lheight > rheight)
            return(lheight+1);
        else return(rheight+1);
    }
}
 
/* Helper function that allocates a new node with the
   given data and NULL left and right pointers. */
struct node* newNode(int data)
{
    struct node* node = (struct node*)
                        malloc(sizeof(struct node));
    node->data = data;
    node->left = NULL;
    node->right = NULL;
 
    return(node);
}
 
/* Driver program to test above functions*/
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);
 
    printf("Level Order traversal of binary tree is \n");
    reverseLevelOrder(root);
 
    return 0;
}


Java




// A recursive java program to print reverse level order traversal
  
// A binary tree node
class Node
{
    int data;
    Node left, right;
      
    Node(int item)
    {
        data = item;
        left = right;
    }
}
  
class BinaryTree
{
    Node root;
  
    /* Function to print REVERSE level order traversal a tree*/
    void reverseLevelOrder(Node node)
    {
        int h = height(node);
        int i;
        for (i = h; i >= 1; i--)
        //THE ONLY LINE DIFFERENT FROM NORMAL LEVEL ORDER
        {
            printGivenLevel(node, i);
        }
    }
  
    /* Print nodes at a given level */
    void printGivenLevel(Node node, int level)
    {
        if (node == null)
            return;
        if (level == 1)
            System.out.print(node.data + " ");
        else if (level > 1)
        {
            printGivenLevel(node.left, level - 1);
            printGivenLevel(node.right, level - 1);
        }
    }
  
    /* Compute the "height" of a tree -- the number of
     nodes along the longest path from the root node
     down to the farthest leaf node.*/
    int height(Node node)
    {
        if (node == null)
            return 0;
        else
        {
            /* compute the height of each subtree */
            int lheight = height(node.left);
            int rheight = height(node.right);
  
            /* use the larger one */
            if (lheight > rheight)
                return (lheight + 1);
            else
                return (rheight + 1);
        }
    }
  
    // Driver program to test above functions
    public static void main(String args[])
    {
        BinaryTree tree = new BinaryTree();
  
        // Let us create trees shown in above diagram
        tree.root = new Node(1);
        tree.root.left = new Node(2);
        tree.root.right = new Node(3);
        tree.root.left.left = new Node(4);
        tree.root.left.right = new Node(5);
          
        System.out.println("Level Order traversal of binary tree is : ");
        tree.reverseLevelOrder(tree.root);
    }
}
  
// This code has been contributed by Mayank Jaiswal


Python




# A recursive Python program to print REVERSE level order traversal
 
# A binary tree node
class Node:
 
    # Constructor to create a new node
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None
 
# Function to print reverse level order traversal
def reverseLevelOrder(root):
    h = height(root)
    for i in reversed(range(1, h+1)):
        printGivenLevel(root,i)
 
# Print nodes at a given level
def printGivenLevel(root, level):
 
    if root is None:
        return
    if level ==1 :
        print root.data,
 
    elif level>1:
        printGivenLevel(root.left, level-1)
        printGivenLevel(root.right, level-1)
 
# Compute the height of a tree-- the number of
# nodes along the longest path from the root node
# down to the farthest leaf node
def height(node):
    if node is None:
        return 0
    else:
 
        # Compute the height of each subtree
        lheight = height(node.left)
        rheight = height(node.right)
 
        # Use the larger one
        if lheight > rheight :
            return lheight + 1
        else:
            return rheight + 1
 
# Driver program to test above function
root = Node(1)
root.left = Node(2)
root.right = Node(3)
root.left.left = Node(4)
root.left.right = Node(5)
 
print "Level Order traversal of binary tree is"
reverseLevelOrder(root)
 
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)


C#




// A recursive C# program to print
// reverse level order traversal
using System;
 
// A binary tree node
class Node
{
    public int data;
    public Node left, right;
         
    public Node(int item)
    {
        data = item;
        left = right;
    }
}
     
class BinaryTree
{
Node root;
 
/* Function to print REVERSE
level order traversal a tree*/
void reverseLevelOrder(Node node)
{
    int h = height(node);
    int i;
    for (i = h; i >= 1; i--)
     
    // THE ONLY LINE DIFFERENT
    // FROM NORMAL LEVEL ORDER
    {
        printGivenLevel(node, i);
    }
}
 
/* Print nodes at a given level */
void printGivenLevel(Node node, int level)
{
    if (node == null)
        return;
    if (level == 1)
        Console.Write(node.data + " ");
    else if (level > 1)
    {
        printGivenLevel(node.left, level - 1);
        printGivenLevel(node.right, level - 1);
    }
}
 
/* Compute the "height" of a tree --
the number of nodes along the longest
path from the root node down to the
farthest leaf node.*/
int height(Node node)
{
    if (node == null)
        return 0;
    else
    {
        /* compute the height of each subtree */
        int lheight = height(node.left);
        int rheight = height(node.right);
 
        /* use the larger one */
        if (lheight > rheight)
            return (lheight + 1);
        else
            return (rheight + 1);
    }
}
 
// Driver Code
static public void Main(String []args)
{
    BinaryTree tree = new BinaryTree();
 
    // Let us create trees shown
    // in above diagram
    tree.root = new Node(1);
    tree.root.left = new Node(2);
    tree.root.right = new Node(3);
    tree.root.left.left = new Node(4);
    tree.root.left.right = new Node(5);
         
    Console.WriteLine("Level Order traversal " +
                        "of binary tree is : ");
    tree.reverseLevelOrder(tree.root);
}
}
     
// This code is contributed
// by Arnab Kundu


Javascript




<script>
 
      // A recursive JavaScript program to print
      // reverse level order traversal
      // A binary tree node
      class Node {
        constructor(item) {
          this.data = item;
          this.left = null;
          this.right = null;
        }
      }
 
      class BinaryTree {
        constructor() {
          this.root = null;
        }
 
        /* Function to print REVERSE
        level order traversal a tree*/
        reverseLevelOrder(node) {
          var h = this.height(node);
          var i;
          for (
            i = h;
            i >= 1;
            i-- // THE ONLY LINE DIFFERENT // FROM NORMAL LEVEL ORDER
          ) {
            this.printGivenLevel(node, i);
          }
        }
 
        /* Print nodes at a given level */
        printGivenLevel(node, level) {
          if (node == null) return;
          if (level == 1) document.write(node.data + " ");
          else if (level > 1) {
            this.printGivenLevel(node.left, level - 1);
            this.printGivenLevel(node.right, level - 1);
          }
        }
 
        /* Compute the "height" of a tree --
         the number of nodes along the longest
         path from the root node down to the
         farthest leaf node.*/
        height(node) {
          if (node == null) return 0;
          else {
            /* compute the height of each subtree */
            var lheight = this.height(node.left);
            var rheight = this.height(node.right);
 
            /* use the larger one */
            if (lheight > rheight) return lheight + 1;
            else return rheight + 1;
          }
        }
      }
      // Driver Code
      var tree = new BinaryTree();
 
      // Let us create trees shown
      // in above diagram
      tree.root = new Node(1);
      tree.root.left = new Node(2);
      tree.root.right = new Node(3);
      tree.root.left.left = new Node(4);
      tree.root.left.right = new Node(5);
 
      document.write(
      "Level Order traversal " + "of binary tree is : <br>"
      );
      tree.reverseLevelOrder(tree.root);
       
 </script>


Output

Level Order traversal of binary tree is 
4 5 2 3 1 






Time Complexity:  O(n^2)
Auxiliary Space: O(h), where h is the height of the tree, this space is due to the recursive call stack.

METHOD 2 (Using Queue and Stack) 
The idea is to use a deque(double-ended queue) to get the reverse level order. A deque allows insertion and deletion at both ends. If we do normal level order traversal and instead of printing a node, push the node to a stack and then print the contents of the deque, we get “5 4 3 2 1” for the above example tree, but the output should be “4 5 2 3 1”. So to get the correct sequence (left to right at every level), we process the children of a node in reverse order, we first push the right subtree to the deque, then process the left subtree.

C++




// A C++ program to print REVERSE level order traversal using stack and queue
// This approach is adopted from following link
#include <bits/stdc++.h>
using namespace std;
 
/* A binary tree node has data, pointer to left and right children */
struct node
{
    int data;
    struct node* left;
    struct node* right;
};
 
/* Given a binary tree, print its nodes in reverse level order */
void reverseLevelOrder(node* root)
{
    stack <node *> S;
    queue <node *> Q;
    Q.push(root);
 
    // Do something like normal level order traversal order. Following are the
    // differences with normal level order traversal
    // 1) Instead of printing a node, we push the node to stack
    // 2) Right subtree is visited before left subtree
    while (Q.empty() == false)
    {
        /* Dequeue node and make it root */
        root = Q.front();
        Q.pop();
        S.push(root);
 
        /* Enqueue right child */
        if (root->right)
            Q.push(root->right); // NOTE: RIGHT CHILD IS ENQUEUED BEFORE LEFT
 
        /* Enqueue left child */
        if (root->left)
            Q.push(root->left);
    }
 
    // Now pop all items from stack one by one and print them
    while (S.empty() == false)
    {
        root = S.top();
        cout << root->data << " ";
        S.pop();
    }
}
 
/* Helper function that allocates a new node with the
   given data and NULL left and right pointers. */
node* newNode(int data)
{
    node* temp = new node;
    temp->data = data;
    temp->left = NULL;
    temp->right = NULL;
 
    return (temp);
}
 
/* Driver program to test above functions*/
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);
 
    cout << "Level Order traversal of binary tree is \n";
    reverseLevelOrder(root);
 
    return 0;
}


Java




// A recursive java program to print reverse level order traversal
// using stack and queue
  
import java.util.LinkedList;
import java.util.Queue;
import java.util.Stack;
  
/* A binary tree node has data, pointer to left and right children */
class Node
{
    int data;
    Node left, right;
  
    Node(int item)
    {
        data = item;
        left = right;
    }
}
  
class BinaryTree
{
    Node root;
  
    /* Given a binary tree, print its nodes in reverse level order */
    void reverseLevelOrder(Node node)
    {
        Stack<Node> S = new Stack();
        Queue<Node> Q = new LinkedList();
        Q.add(node);
  
        // Do something like normal level order traversal order.Following
        // are the differences with normal level order traversal
        // 1) Instead of printing a node, we push the node to stack
        // 2) Right subtree is visited before left subtree
        while (Q.isEmpty() == false)
        {
            /* Dequeue node and make it root */
            node = Q.peek();
            Q.remove();
            S.push(node);
  
            /* Enqueue right child */
            if (node.right != null)
                // NOTE: RIGHT CHILD IS ENQUEUED BEFORE LEFT
                Q.add(node.right);
                 
            /* Enqueue left child */
            if (node.left != null)
                Q.add(node.left);
        }
  
        // Now pop all items from stack one by one and print them
        while (S.empty() == false)
        {
            node = S.peek();
            System.out.print(node.data + " ");
            S.pop();
        }
    }
  
    // Driver program to test above functions
    public static void main(String args[])
    {
        BinaryTree tree = new BinaryTree();
  
        // Let us create trees shown in above diagram
        tree.root = new Node(1);
        tree.root.left = new Node(2);
        tree.root.right = new Node(3);
        tree.root.left.left = new Node(4);
        tree.root.left.right = new Node(5);
        tree.root.right.left = new Node(6);
        tree.root.right.right = new Node(7);
  
        System.out.println("Level Order traversal of binary tree is :");
        tree.reverseLevelOrder(tree.root);
  
    }
}
  
// This code has been contributed by Mayank Jaiswal


Python




# Python program to print REVERSE level order traversal using
# stack and queue
 
from collections import deque
# A binary tree node
 
 
class Node:
 
    # Constructor to create a new node
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None
 
 
# Given a binary tree, print its nodes in reverse level order
 
 
def reverseLevelOrder(root):
  # we can use a double ended queue which provides O(1) insert at the beginning
  # using the appendleft method
  # we do the regular level order traversal but instead of processing the
  # left child first we process the right child first and the we process the left child
  # of the current Node
  # we can do this One pass reduce the space usage not in terms of complexity but intuitively
 
    q = deque()
    q.append(root)
    ans = deque()
    while q:
        node = q.popleft()
        if node is None:
            continue
 
        ans.appendleft(node.data)
 
        if node.right:
            q.append(node.right)
 
        if node.left:
            q.append(node.left)
 
    return ans
 
 
# Driver program to test above function
root = Node(1)
root.left = Node(2)
root.right = Node(3)
root.left.left = Node(4)
root.left.right = Node(5)
root.right.left = Node(6)
root.right.right = Node(7)
 
print "Level Order traversal of binary tree is"
deq = reverseLevelOrder(root)
for key in deq:
    print (key),
 
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)


C#




// A recursive C# program to print reverse
// level order traversal using stack and queue
using System.Collections.Generic;
using System;
 
/* A binary tree node has data,
pointer to left and right children */
public class Node
{
    public int data;
    public Node left, right;
 
    public Node(int item)
    {
        data = item;
        left = right;
    }
}
 
public class BinaryTree
{
    Node root;
 
    /* Given a binary tree, print its
    nodes in reverse level order */
    void reverseLevelOrder(Node node)
    {
        Stack<Node> S = new Stack<Node>();
        Queue<Node> Q = new Queue<Node>();
        Q.Enqueue(node);
 
        // Do something like normal level
        // order traversal order.Following
        // are the differences with normal
        // level order traversal
        // 1) Instead of printing a node, we push the node to stack
        // 2) Right subtree is visited before left subtree
        while (Q.Count>0)
        {
            /* Dequeue node and make it root */
            node = Q.Peek();
            Q.Dequeue();
            S.Push(node);
 
            /* Enqueue right child */
            if (node.right != null)
                // NOTE: RIGHT CHILD IS ENQUEUED BEFORE LEFT
                Q.Enqueue(node.right);
                 
            /* Enqueue left child */
            if (node.left != null)
                Q.Enqueue(node.left);
        }
 
        // Now pop all items from stack
        // one by one and print them
        while (S.Count>0)
        {
            node = S.Peek();
            Console.Write(node.data + " ");
            S.Pop();
        }
    }
 
    // Driver code
    public static void Main()
    {
        BinaryTree tree = new BinaryTree();
 
        // Let us create trees shown in above diagram
        tree.root = new Node(1);
        tree.root.left = new Node(2);
        tree.root.right = new Node(3);
        tree.root.left.left = new Node(4);
        tree.root.left.right = new Node(5);
        tree.root.right.left = new Node(6);
        tree.root.right.right = new Node(7);
 
        Console.WriteLine("Level Order traversal of binary tree is :");
        tree.reverseLevelOrder(tree.root);
 
    }
}
 
/* This code contributed by PrinciRaj1992 */


Javascript




<script>
 
// A recursive javascript program to print
// reverse level order traversal
// using stack and queue
 
/* A binary tree node has data, pointer to left
and right children */
class Node
{
    constructor(item)
    {
        this.data = item;
        this.left = this.right=null;
    }
}
 
let root;
 
/* Given a binary tree, print its nodes in reverse level order */
function reverseLevelOrder(node)
{   
        let S = [];
        let Q = [];
        Q.push(node);
   
        // Do something like normal
        // level order traversal order.Following
        // are the differences with normal
        // level order traversal
        // 1) Instead of printing a node,
        // we push the node to stack
        // 2) Right subtree is visited before left subtree
        while (Q.length != 0)
        {
            /* Dequeue node and make it root */
            node = Q[0];
            Q.shift();
            S.push(node);
   
            /* Enqueue right child */
            if (node.right != null)
                // NOTE: RIGHT CHILD IS ENQUEUED BEFORE LEFT
                Q.push(node.right);
                  
            /* Enqueue left child */
            if (node.left != null)
                Q.push(node.left);
        }
   
        // Now pop all items from stack
        // one by one and print them
        while (S.length != 0)
        {
            node = S.pop();
            document.write(node.data + " ");
             
        }
}
 
// Driver program to test above functions
// Let us create trees shown in above diagram
root = new Node(1);
root.left = new Node(2);
root.right = new Node(3);
root.left.left = new Node(4);
root.left.right = new Node(5);
root.right.left = new Node(6);
root.right.right = new Node(7);
 
document.write("Level Order traversal of binary tree is :<br>");
reverseLevelOrder(root);
 
// This code is contributed by avanitrachhadiya2155
 
</script>


Output

Level Order traversal of binary tree is 
4 5 6 7 2 3 1 






Time Complexity: O(n), where n is the number of nodes in the binary tree. 
Auxiliary Space: O(n), for stack and queue.

Method 3: ( Using a Hash_Map)

The basic idea behind this approach is to use a hashmap to store the nodes at each level of the binary tree, and then iterate over the hashmap in reverse order of the levels to obtain the reverse level order traversal.

Follow the steps to implement above idea:

  1. Define a Node struct to represent a binary tree node.
  2. Define a recursive function addNodesToMap that takes a binary tree node, a level, and a reference to an unordered map, and adds the node to the vector of nodes at its level in the unordered map. This function should then recursively call itself on the left and right subtrees.
  3. Define the main reverseLevelOrder function that takes the root of the binary tree as input, and returns a vector containing the nodes in reverse level order.
  4. Inside the reverseLevelOrder function, create an unordered map to store the nodes at each level of the binary tree.
  5. Call the addNodesToMap function on the root of the binary tree, with level 0 and a reference to the unordered map, to populate the map with nodes.
  6. Iterate over the unordered map in reverse order of the levels, and add the nodes to the result vector in the order they appear in the vectors in the unordered map.
  7. Return the result vector containing the nodes in reverse level order.

Below is the implementation:

C++




// C++ code to implement the hash_map approach
#include <bits/stdc++.h>
#include <unordered_map>
using namespace std;
 
// Definition of a binary tree node
struct Node {
    int data;
    Node* left;
    Node* right;
    Node(int val)
    {
        data = val;
        left = right = nullptr;
    }
};
 
// Recursive function to traverse the binary
// tree and add nodes to the hashmap
void addNodesToMap(
    Node* node, int level,
    unordered_map<int, vector<int> >& nodeMap)
{
    if (node == nullptr) {
        return;
    }
    // Add the current node to the vector of
    // nodes at its level in the hashmap
    nodeMap[level].push_back(node->data);
    // Recursively traverse the left and
    // right subtrees
    addNodesToMap(node->left, level + 1, nodeMap);
    addNodesToMap(node->right, level + 1, nodeMap);
}
 
vector<int> reverseLevelOrder(Node* root)
{
    vector<int> result;
    // Create an unordered_map to store the
    // nodes at each level of the binary tree
    unordered_map<int, vector<int> > nodeMap;
    // Traverse the binary tree recursively and
    // add nodes to the hashmap
    addNodesToMap(root, 0, nodeMap);
    // Iterate over the hashmap in reverse order of the
    // levels and add nodes to the result vector
    for (int level = nodeMap.size() - 1; level >= 0;
         level--) {
        vector<int> nodesAtLevel = nodeMap[level];
        for (int i = 0; i < nodesAtLevel.size(); i++) {
            result.push_back(nodesAtLevel[i]);
        }
    }
    return result;
}
 
// Driver code
int main()
{
    // Create the binary 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);
    // Find the reverse level order traversal
    // of the binary tree
    vector<int> result = reverseLevelOrder(root);
    cout << "Level Order traversal of binary tree is:"
         << endl;
    // Print the result
    for (int i = 0; i < result.size(); i++) {
        cout << result[i] << " ";
    }
    cout << endl;
    return 0;
}
//This code is contributed by Veerendra_Singh_Rajpoot


Java




import java.util.*;
 
// Definition of a binary tree node
class Node {
    int data;
    Node left;
    Node right;
 
    Node(int val) {
        data = val;
        left = right = null;
    }
}
 
class Main {
    // Recursive function to traverse the binary
    // tree and add nodes to the hashmap
    static void addNodesToMap(Node node, int level, Map<Integer, List<Integer>> nodeMap) {
        if (node == null) {
            return;
        }
        // Add the current node to the list of
        // nodes at its level in the hashmap
        if (!nodeMap.containsKey(level)) {
            nodeMap.put(level, new ArrayList<>());
        }
        nodeMap.get(level).add(node.data);
        // Recursively traverse the left and
        // right subtrees
        addNodesToMap(node.left, level + 1, nodeMap);
        addNodesToMap(node.right, level + 1, nodeMap);
    }
 
    static List<Integer> reverseLevelOrder(Node root) {
        List<Integer> result = new ArrayList<>();
        // Create a map to store the nodes at each level of the binary tree
        Map<Integer, List<Integer>> nodeMap = new HashMap<>();
        // Traverse the binary tree recursively and
        // add nodes to the hashmap
        addNodesToMap(root, 0, nodeMap);
        // Iterate over the hashmap in reverse order of the
        // levels and add nodes to the result list
        for (int level = nodeMap.size() - 1; level >= 0; level--) {
            List<Integer> nodesAtLevel = nodeMap.get(level);
            for (int i = 0; i < nodesAtLevel.size(); i++) {
                result.add(nodesAtLevel.get(i));
            }
        }
        return result;
    }
 
    // Driver code
    public static void main(String[] args) {
        // Create the binary 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);
        // Find the reverse level order traversal
        // of the binary tree
        List<Integer> result = reverseLevelOrder(root);
        System.out.println("Level Order traversal of binary tree is:");
        // Print the result
        for (int i = 0; i < result.size(); i++) {
            System.out.print(result.get(i) + " ");
        }
        System.out.println();
    }
}
// This code is contributed by Veerendra_Singh_Rajpoot


Python




class Node:
    def __init__(self, val):
        self.data = val
        self.left = None
        self.right = None
# Recursive function to traverse the binary
def addNodesToMap(node, level, nodeMap):
    if node is None:
        return
    if level in nodeMap:
        nodeMap[level].append(node.data)
    else:
        nodeMap[level] = [node.data]
    # Recursively traverse the left and right subtrees
    addNodesToMap(node.left, level + 1, nodeMap)
    addNodesToMap(node.right, level + 1, nodeMap)
def GFG(root):
    result = []
    nodeMap = {}
    # Traverse the binary tree recursively
    addNodesToMap(root, 0, nodeMap)
    # Iterate over the dictionary in reverse order
    for level in range(len(nodeMap) - 1, -1, -1):
        nodesAtLevel = nodeMap[level]
        result.extend(nodesAtLevel)
    return result
# Driver code
if __name__ == "__main__":
    # Create the binary tree
    root = Node(10)
    root.left = Node(20)
    root.right = Node(30)
    root.left.left = Node(40)
    root.left.right = Node(60)
    # Find the reverse level order traversal of the binary tree
    result = GFG(root)
    print("Level Order traversal of binary tree is:")
    # Print the result without brackets
    print(" ".join(map(str, result)))


C#




using System;
using System.Collections.Generic;
 
// Definition of a binary tree node
class Node {
    public int data;
    public Node left;
    public Node right;
    public Node(int val)
    {
        data = val;
        left = right = null;
    }
}
 
class Program {
    // Recursive function to traverse the binary
    // tree and add nodes to the hashmap
    static void
    AddNodesToMap(Node node, int level,
                  Dictionary<int, List<int> > nodeMap)
    {
        if (node == null) {
            return;
        }
        // Add the current node to the list of
        // nodes at its level in the hashmap
        if (!nodeMap.ContainsKey(level)) {
            nodeMap[level] = new List<int>();
        }
        nodeMap[level].Add(node.data);
        // Recursively traverse the left and
        // right subtrees
        AddNodesToMap(node.left, level + 1, nodeMap);
        AddNodesToMap(node.right, level + 1, nodeMap);
    }
 
    static List<int> ReverseLevelOrder(Node root)
    {
        List<int> result = new List<int>();
        // Create a dictionary to store the
        // nodes at each level of the binary tree
        Dictionary<int, List<int> > nodeMap
            = new Dictionary<int, List<int> >();
        // Traverse the binary tree recursively and
        // add nodes to the dictionary
        AddNodesToMap(root, 0, nodeMap);
        // Iterate over the dictionary in reverse order of
        // the levels and add nodes to the result list
        for (int level = nodeMap.Count - 1; level >= 0;
             level--) {
            List<int> nodesAtLevel = nodeMap[level];
            for (int i = 0; i < nodesAtLevel.Count; i++) {
                result.Add(nodesAtLevel[i]);
            }
        }
        return result;
    }
 
    // Driver code
    static void Main()
    {
        // Create the binary 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);
        // Find the reverse level order traversal
        // of the binary tree
        List<int> result = ReverseLevelOrder(root);
        Console.WriteLine(
            "Level Order traversal of binary tree is:");
        // Print the result
        foreach(int val in result)
        {
            Console.Write(val + " ");
        }
        Console.WriteLine();
    }
}


Javascript




// Definition of a binary tree node
class Node {
    constructor(val) {
        this.data = val;
        this.left = null;
        this.right = null;
    }
}
 
// Recursive function to traverse the binary
// tree and add nodes to the hashmap
function addNodesToMap(node, level, nodeMap) {
    if (node === null) {
        return;
    }
    // Add the current node to the vector of
    // nodes at its level in the hashmap
    if (!nodeMap[level]) {
        nodeMap[level] = [];
    }
    nodeMap[level].push(node.data);
    // Recursively traverse the left and
    // right subtrees
    addNodesToMap(node.left, level + 1, nodeMap);
    addNodesToMap(node.right, level + 1, nodeMap);
}
 
function reverseLevelOrder(root) {
    const result = [];
    // Create an object to store the nodes at each level of the binary tree
    const nodeMap = {};
    // Traverse the binary tree recursively and
    // add nodes to the hashmap
    addNodesToMap(root, 0, nodeMap);
    // Iterate over the hashmap in reverse order of the
    // levels and add nodes to the result array
    const levels = Object.keys(nodeMap).map(Number);
    levels.sort((a, b) => b - a); // Sort levels in reverse order
    for (const level of levels) {
        result.push(...nodeMap[level]);
    }
    return result;
}
 
// Driver code
// Create the binary tree
const 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);
 
// Find the reverse level order traversal
// of the binary tree
const result = reverseLevelOrder(root);
console.log("Level Order traversal of binary tree is:");
// Print the result
console.log(result.join(" "));
// This code is contributed by Veerendra_Singh_Rajpoot


Output

Level Order traversal of binary tree is:
40 60 20 30 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.



Last Updated : 04 Oct, 2023
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