Zig-Zag traversal of a Binary Tree using Recursion

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

Examples:

Input : 
          7
         / \
        6   5
       /   /
      4   3
     /     \
    2       1
Output : 7 5 6 4 3 1 2

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

We have already discussed the zigzag traversal using an iterative approach, in this post we will solve it using Recursion.

Recursive Approach: The idea is to traverse the tree in a Level Order manner but in a slightly different manner. We will use a variable flag and initially set it’s value to zero. As we complete the level order traversal of the tree, from right to left we will set the value of flag to one, so that next time we can traverse the Tree from left to right and as we complete the traversal we will set it’s value back to zero. 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 zigzag
// traversal of a binary tree 
// using Recursion
  
#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 Tree 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;
      
    int lheight = heightOfTree(root->left);
    int rheight = heightOfTree(root->right);
      
    return max(lheight + 1 ,rheight + 1);
}
  
// Function to print nodes from right to left
void printRightToLeft(struct node* root ,int level)
{
    if(root == NULL)
    return;
      
    if(level == 1)
    cout << root->data << " " ;
      
    else if(level > 1)
    {
        printRightToLeft(root->right ,level - 1);
        printRightToLeft(root->left ,level - 1);
    }
}
  
// Function to print nodes from left to right
void printLeftToRight(struct node* root ,int level)
{
    if(root == NULL)
    return;
      
    if(level == 1)
    cout << root->data << " " ;
      
    else if(level > 1)
    {
        printLeftToRight(root->left ,level - 1);
        printLeftToRight(root->right ,level - 1);
    }
}
  
// Function to print Reverse ZigZag of 
// a Binary tree
void printZigZag(struct node* root )
{
    // Flag is used to mark the change
    // in level
    int flag = 0;
      
    // Height of tree
    int height = heightOfTree(root);
      
    for(int i = 1 ; i <= height ; i++)
    {
        // If flag value is one print nodes
        // from right to left
        if(flag == 1)
        {
            printRightToLeft(root ,i);
              
            // Mark flag as zero so that next time
            // nodes are printed from left to right
            flag = 0;
        }
        // If flag is zero print nodes
        // from left to right 
        else if(flag == 0)
        {
            printLeftToRight(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(7); 
    root->left = newNode(4); 
    root->right = newNode(5); 
    root->left->left = newNode(9); 
    root->right->right = newNode(10); 
    root->left->left->left = newNode(6); 
    root->left->left->right = newNode(11); 
      
    printZigZag(root); 
  
    return 0; 
}

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Java

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// Java program to print zigzag 
// traversal of a binary tree 
// using Recursion 
  
class GfG 
  
// Binary tree node 
static class node 
    node left; 
    node right; 
    int data; 
}
  
// Function to create a new 
// Binary Tree 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
      
    int lheight = heightOfTree(root.left); 
    int rheight = heightOfTree(root.right); 
      
    return Math.max(lheight + 1 ,rheight + 1); 
  
// Function to print nodes from right to left 
static void printRightToLeft(node root ,int level) 
    if(root == null
    return
      
    if(level == 1
    System.out.print(root.data + " ") ; 
      
    else if(level > 1
    
        printRightToLeft(root.right ,level - 1); 
        printRightToLeft(root.left ,level - 1); 
    
  
// Function to print nodes from left to right 
static void printLeftToRight(node root ,int level) 
    if(root == null
    return
      
    if(level == 1
    System.out.print(root.data + " ") ; 
      
    else if(level > 1
    
        printLeftToRight(root.left ,level - 1); 
        printLeftToRight(root.right ,level - 1); 
    
  
// Function to print Reverse ZigZag of 
// a Binary tree 
static void printZigZag(node root ) 
    // Flag is used to mark the change 
    // in level 
    int flag = 0
      
    // Height of tree 
    int height = heightOfTree(root); 
      
    for(int i = 1 ; i <= height ; i++) 
    
        // If flag value is one print nodes 
        // from right to left 
        if(flag == 1
        
            printRightToLeft(root ,i); 
              
            // Mark flag as zero so that next time 
            // nodes are printed from left to right 
            flag = 0
        
        // If flag is zero print nodes 
        // from left to right 
        else if(flag == 0
        
            printLeftToRight(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(7); 
    root.left = newNode(4); 
    root.right = newNode(5); 
    root.left.left = newNode(9); 
    root.right.right = newNode(10); 
    root.left.left.left = newNode(6); 
    root.left.left.right = newNode(11); 
      
    printZigZag(root); 
}
  
// This code is contributed by Prerna Saini.

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Output:

7 5 4 9 10 11 6        


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