Reverse zigzag Traversal of a Binary Tree

Given a Binary Tree, the task is to print the Reverse zigzag Order of the tree.

Examples:

Input:
     1
   /  \
  2    3
 / \    \
4   5    6
Output: 6 5 4 2 3 1

Input:
      5
     / \
    9   3
   /     \
  6       4
 / \
8   7
Output: 7 8 6 4 3 9 5

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach: The idea is to traverse the tree in a Reverse Level Order manner but with a slight modification. We will use a variable flag and initially set it’s value to one. As we complete the reverse level order traversal of the tree, from right to left we will set the value of flag to zero, so that next time we traverse the Tree from left to right and as we complete the traversal we set it’s value back to one. We will repeat this whole step until we have traversed the Binary Tree completely.

Below is the implementation of the above approach:

C++

// C++ program to print reverse 
// zigzag order of binary tree 
#include <bits/stdc++.h> 
using namespace std; 
  
// Binary tree node 
struct node { 
    struct node* left; 
    struct node* right; 
    int data; 
}; 
  
// Function to create a new 
// Binary node 
struct node* newNode(int data) 
{ 
    struct node* temp = new node; 
  
    temp->data = data; 
    temp->left = NULL; 
    temp->right = NULL; 
  
    return temp; 
} 
  
// Recursive Function to find height 
// of binary tree 
int HeightOfTree(struct node* root) 
{ 
    if (root == NULL) 
        return 0; 
  
    // Compute the height of each subtree 
    int lheight = HeightOfTree(root->left); 
    int rheight = HeightOfTree(root->right); 
  
    // Return the maximum of two 
    return max(lheight + 1, rheight + 1); 
} 
  
// Function to Print nodes from right to left 
void Print_Level_Right_To_Left(struct node* root, int level) 
{ 
    if (root == NULL) 
        return; 
  
    if (level == 1) 
        cout << root->data << " "; 
  
    else if (level > 1) { 
        Print_Level_Right_To_Left(root->right, level - 1); 
        Print_Level_Right_To_Left(root->left, level - 1); 
    } 
} 
  
// Function to Print nodes from left to right 
void Print_Level_Left_To_Right(struct node* root, int level) 
{ 
    if (root == NULL) 
        return; 
  
    if (level == 1) 
        cout << root->data << " "; 
  
    else if (level > 1) { 
        Print_Level_Left_To_Right(root->left, level - 1); 
        Print_Level_Left_To_Right(root->right, level - 1); 
    } 
} 
  
// Function to print Reverse zigzag of 
// a Binary tree 
void PrintReverseZigZag(struct node* root) 
{ 
    // Flag is used to mark the change 
    // in level 
    int flag = 1; 
  
    // Height of tree 
    int height = HeightOfTree(root); 
  
    for (int i = height; i >= 1; i--) { 
  
        // If flag value is one print nodes 
        // from right to left 
        if (flag == 1) { 
            Print_Level_Right_To_Left(root, i); 
  
            // Mark flag as zero so that next time 
            // tree is traversed from left to right 
            flag = 0; 
        } 
  
        // If flag is zero print nodes 
        // from left to right 
        else if (flag == 0) { 
            Print_Level_Left_To_Right(root, i); 
  
            // Mark flag as one so that next time 
            // nodes are printed from right to left 
            flag = 1; 
        } 
    } 
} 
  
// Driver code 
int main() 
{ 
    struct node* root = newNode(5); 
    root->left = newNode(9); 
    root->right = newNode(3); 
    root->left->left = newNode(6); 
    root->right->right = newNode(4); 
    root->left->left->left = newNode(8); 
    root->left->left->right = newNode(7); 
  
    PrintReverseZigZag(root); 
  
    return 0; 
} 

Java

// Java program to print reverse  
// zigzag order of binary tree  
class GfG 
{ 
  
// Binary tree node  
static class node  
{  
    node left;  
    node right;  
    int data;  
} 
  
// Function to create a new  
// Binary node  
static node newNode(int data)  
{  
    node temp = new node();  
  
    temp.data = data;  
    temp.left = null;  
    temp.right = null;  
  
    return temp;  
}  
  
// Recursive Function to find height  
// of binary tree  
static int HeightOfTree(node root)  
{  
    if (root == null)  
        return 0;  
  
    // Compute the height of each subtree  
    int lheight = HeightOfTree(root.left);  
    int rheight = HeightOfTree(root.right);  
  
    // Return the maximum of two  
    return Math.max(lheight + 1, rheight + 1);  
}  
  
// Function to Print nodes from right to left  
static void Print_Level_Right_To_Left(node root, int level)  
{  
    if (root == null)  
        return;  
  
    if (level == 1)  
        System.out.print(root.data + " ");  
  
    else if (level > 1)  
    {  
        Print_Level_Right_To_Left(root.right, level - 1);  
        Print_Level_Right_To_Left(root.left, level - 1);  
    }  
}  
  
// Function to Print nodes from left to right  
static void Print_Level_Left_To_Right(node root, int level)  
{  
    if (root == null)  
        return;  
  
    if (level == 1)  
        System.out.print(root.data + " ");  
  
    else if (level > 1) 
    {  
        Print_Level_Left_To_Right(root.left, level - 1);  
        Print_Level_Left_To_Right(root.right, level - 1);  
    }  
}  
  
// Function to print Reverse zigzag of  
// a Binary tree  
static void PrintReverseZigZag(node root)  
{  
    // Flag is used to mark the change  
    // in level  
    int flag = 1;  
  
    // Height of tree  
    int height = HeightOfTree(root);  
  
    for (int i = height; i >= 1; i--)  
    {  
  
        // If flag value is one print nodes  
        // from right to left  
        if (flag == 1)  
        {  
            Print_Level_Right_To_Left(root, i);  
  
            // Mark flag as zero so that next time  
            // tree is traversed from left to right  
            flag = 0;  
        }  
  
        // If flag is zero print nodes  
        // from left to right  
        else if (flag == 0)  
        {  
            Print_Level_Left_To_Right(root, i);  
  
            // Mark flag as one so that next time  
            // nodes are printed from right to left  
            flag = 1;  
        }  
    }  
}  
  
// Driver code  
public static void main(String[] args)  
{  
    node root = newNode(5);  
    root.left = newNode(9);  
    root.right = newNode(3);  
    root.left.left = newNode(6);  
    root.right.right = newNode(4);  
    root.left.left.left = newNode(8);  
    root.left.left.right = newNode(7);  
  
    PrintReverseZigZag(root);  
} 
}  
  
// This code is contributed by Prerna Saini. 

Python3

# Python3 program to print reverse  
# zigzag order of binary tree  
  
# Binary tree node  
class Node:  
      
    def __init__(self, data): 
        self.data = data 
        self.left = None
        self.right = None
      
# Recursive Function to find  
# height of binary tree  
def HeightOfTree(root):  
  
    if root == None: 
        return 0
  
    # Compute the height of each subtree  
    lheight = HeightOfTree(root.left)  
    rheight = HeightOfTree(root.right)  
  
    # Return the maximum of two  
    return max(lheight + 1, rheight + 1)  
  
# Function to Print nodes from right to left  
def Print_Level_Right_To_Left(root, level):  
  
    if root == None:  
        return
  
    if level == 1: 
        print(root.data, end = " ")  
  
    elif level > 1: 
        Print_Level_Right_To_Left(root.right, 
                                  level - 1)  
        Print_Level_Right_To_Left(root.left, 
                                  level - 1)  
  
# Function to Print nodes from left to right  
def Print_Level_Left_To_Right(root, level):  
  
    if root == None:  
        return
  
    if level == 1:  
        print(root.data, end = " ")  
  
    elif level > 1:  
        Print_Level_Left_To_Right(root.left,     
                                  level - 1)  
        Print_Level_Left_To_Right(root.right, 
                                  level - 1)  
      
# Function to print Reverse  
# zigzag of a Binary tree  
def PrintReverseZigZag(root):  
  
    # Flag is used to mark the  
    # change in level  
    flag = 1
  
    # Height of tree  
    height = HeightOfTree(root)  
  
    for i in range(height, 0, -1):  
  
        # If flag value is one print  
        # nodes from right to left  
        if flag == 1:  
            Print_Level_Right_To_Left(root, i)  
  
            # Mark flag as zero so that next time  
            # tree is traversed from left to right  
            flag = 0
          
        # If flag is zero print nodes  
        # from left to right  
        elif flag == 0:  
            Print_Level_Left_To_Right(root, i)  
  
            # Mark flag as one so that next time  
            # nodes are printed from right to left  
            flag = 1
  
# Driver code  
if __name__ == "__main__": 
  
    root = Node(5)  
    root.left = Node(9)  
    root.right = Node(3)  
    root.left.left = Node(6)  
    root.right.right = Node(4)  
    root.left.left.left = Node(8)  
    root.left.left.right = Node(7)  
  
    PrintReverseZigZag(root)  
  
# This code is contributed by Rituraj Jain 

C#

// C# program to print reverse  
// zigzag order of binary tree  
using System; 
  
class GfG 
{ 
  
// Binary tree node  
public class node  
{  
    public node left;  
    public node right;  
    public int data;  
} 
  
// Function to create a new  
// Binary node  
static node newNode(int data)  
{  
    node temp = new node();  
  
    temp.data = data;  
    temp.left = null;  
    temp.right = null;  
  
    return temp;  
}  
  
// Recursive Function to find height  
// of binary tree  
static int HeightOfTree(node root)  
{  
    if (root == null)  
        return 0;  
  
    // Compute the height of each subtree  
    int lheight = HeightOfTree(root.left);  
    int rheight = HeightOfTree(root.right);  
  
    // Return the maximum of two  
    return Math.Max(lheight + 1, rheight + 1);  
}  
  
// Function to Print nodes from right to left  
static void Print_Level_Right_To_Left(node root, int level)  
{  
    if (root == null)  
        return;  
  
    if (level == 1)  
        Console.Write(root.data + " ");  
  
    else if (level > 1)  
    {  
        Print_Level_Right_To_Left(root.right, level - 1);  
        Print_Level_Right_To_Left(root.left, level - 1);  
    }  
}  
  
// Function to Print nodes from left to right  
static void Print_Level_Left_To_Right(node root, int level)  
{  
    if (root == null)  
        return;  
  
    if (level == 1)  
        Console.Write(root.data + " ");  
  
    else if (level > 1) 
    {  
        Print_Level_Left_To_Right(root.left, level - 1);  
        Print_Level_Left_To_Right(root.right, level - 1);  
    }  
}  
  
// Function to print Reverse zigzag of  
// a Binary tree  
static void PrintReverseZigZag(node root)  
{  
    // Flag is used to mark the change  
    // in level  
    int flag = 1;  
  
    // Height of tree  
    int height = HeightOfTree(root);  
  
    for (int i = height; i >= 1; i--)  
    {  
  
        // If flag value is one print nodes  
        // from right to left  
        if (flag == 1)  
        {  
            Print_Level_Right_To_Left(root, i);  
  
            // Mark flag as zero so that next time  
            // tree is traversed from left to right  
            flag = 0;  
        }  
  
        // If flag is zero print nodes  
        // from left to right  
        else if (flag == 0)  
        {  
            Print_Level_Left_To_Right(root, i);  
  
            // Mark flag as one so that next time  
            // nodes are printed from right to left  
            flag = 1;  
        }  
    }  
}  
  
// Driver code  
public static void Main(String[] args)  
{  
    node root = newNode(5);  
    root.left = newNode(9);  
    root.right = newNode(3);  
    root.left.left = newNode(6);  
    root.right.right = newNode(4);  
    root.left.left.left = newNode(8);  
    root.left.left.right = newNode(7);  
  
    PrintReverseZigZag(root);  
} 
} 
  
/* This code contributed by PrinciRaj1992 */

Output:

7 8 6 4 3 9 5

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Recommended: Please try your approach on {IDE} first, before moving on to the solution.

C++

Java

Python3

C#

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