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

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++

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// 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;
}

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Java

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// 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.

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Output:

7 8 6 4 3 9 5


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