Reverse alternate levels of a perfect binary tree

Given a Perfect Binary Tree, reverse the alternate level nodes of the binary tree. 

Given tree: 
               a
            /     \
           b       c
         /  \     /  \
        d    e    f    g
       / \  / \  / \  / \
       h  i j  k l  m  n  o 

Modified tree:
               a
            /     \
           c       b
         /  \     /  \
        d    e    f    g
       / \  / \  / \  / \
      o  n m  l k  j  i  h 

Method 1 (Simple):

A simple solution is to do the following steps. 

1. Access nodes level by level. 
2. If the current level is odd, then store nodes of this level in an array. 
3. Reverse the array and store elements back in the tree.

Method 2 (Using Two Traversals):

Another method is to do two inorder traversals. The following are the steps to be followed. 

1. Traverse the given tree in inorder fashion and store all odd level nodes in an auxiliary array. For the above example given tree, contents of array become {h, i, b, j, k, l, m, c, n, o}
2. Reverse the array. The array now becomes {o, n, c, m, l, k, j, b, i, h}
3. Traverse the tree again inorder fashion. While traversing the tree, one by one take elements from array and store elements from an array to every odd level traversed node. 

For the above example, we traverse ‘h’ first in the above array and replace ‘h’ with ‘o’. Then we traverse ‘i’ and replace it with n. 

Following is the implementation of the above algorithm.

C++

// C++ program to reverse alternate
// levels of a binary tree
#include<bits/stdc++.h>
#define MAX 100
using namespace std;

// A Binary Tree node
struct Node
{
    char data;
    struct Node *left, *right;
};

// A utility function to create a
// new Binary Tree Node
struct Node *newNode(char item)
{
    struct Node *temp =  new Node;
    temp->data = item;
    temp->left = temp->right = NULL;
    return temp;
}

// Function to store nodes of
// alternate levels in an array
void storeAlternate(Node *root, char arr[],
                        int *index, int l)
{
    // Base case
    if (root == NULL) return;

    // Store elements of left subtree
    storeAlternate(root->left, arr, index, l+1);

    // Store this node only if this is a odd level node
    if (l%2 != 0)
    {
        arr[*index] = root->data;
        (*index)++;
    }

    // Store elements of right subtree
    storeAlternate(root->right, arr, index, l+1);
}

// Function to modify Binary Tree
// (All odd level nodes are
// updated by taking elements from
// array in inorder fashion)
void modifyTree(Node *root, char arr[],
                           int *index, int l)
{
    // Base case
    if (root == NULL) return;

    // Update nodes in left subtree
    modifyTree(root->left, arr, index, l+1);

    // Update this node only if this
    // is an odd level node
    if (l%2 != 0)
    {
        root->data = arr[*index];
        (*index)++;
    }

    // Update nodes in right subtree
    modifyTree(root->right, arr, index, l+1);
}

// A utility function to reverse an array from index
// 0 to n-1
void reverse(char arr[], int n)
{
    int l = 0, r = n-1;
    while (l < r)
    {
        int temp = arr[l];
        arr[l] = arr[r];
        arr[r] = temp;
        l++; r--;
    }
}

// The main function to reverse
// alternate nodes of a binary tree
void reverseAlternate(struct Node *root)
{
    // Create an auxiliary array to store
    // nodes of alternate levels
    char *arr = new char[MAX];
    int index = 0;

    // First store nodes of alternate levels
    storeAlternate(root, arr, &index, 0);

    // Reverse the array
    reverse(arr, index);

    // Update tree by taking elements from array
    index = 0;
    modifyTree(root, arr, &index, 0);
}

// A utility function to print indorder traversal of a
// binary tree
void printInorder(struct Node *root)
{
    if (root == NULL) return;
    printInorder(root->left);
    cout << root->data << " ";
    printInorder(root->right);
}

// Driver Program to test above functions
int main()
{
    struct Node *root = newNode('a');
    root->left = newNode('b');
    root->right = newNode('c');
    root->left->left = newNode('d');
    root->left->right = newNode('e');
    root->right->left = newNode('f');
    root->right->right = newNode('g');
    root->left->left->left = newNode('h');
    root->left->left->right = newNode('i');
    root->left->right->left = newNode('j');
    root->left->right->right = newNode('k');
    root->right->left->left = newNode('l');
    root->right->left->right = newNode('m');
    root->right->right->left = newNode('n');
    root->right->right->right = newNode('o');

    cout << "Inorder Traversal of given tree\n";
    printInorder(root);

    reverseAlternate(root);

    cout << "\n\nInorder Traversal of modified tree\n";
    printInorder(root);

    return 0;
}

Java

// Java program to reverse alternate
// levels of  perfect binary tree
// A binary tree node
class Node {

    char data;
    Node left, right;

    Node(char item) {
        data = item;
        left = right = null;
    }
}

// class to access index value by reference
class Index {

    int index;
}

class BinaryTree {

    Node root;
    Index index_obj = new Index();

    // function to store alternate levels in a tree
    void storeAlternate(Node node, char arr[],
                         Index index, int l)
    {
        // base case
        if (node == null) {
            return;
        }
        // store elements of left subtree
        storeAlternate(node.left, arr, index, l + 1);

        // store this node only if level is odd
        if (l % 2 != 0) {
            arr[index.index] = node.data;
            index.index++;
        }

        storeAlternate(node.right, arr, index, l + 1);
    }

    // Function to modify Binary Tree
    // (All odd level nodes are
    // updated by taking elements from
    // array in inorder fashion)
    void modifyTree(Node node, char arr[],
                      Index index, int l)
    {

        // Base case
        if (node == null) {
            return;
        }

        // Update nodes in left subtree
        modifyTree(node.left, arr, index, l + 1);

        // Update this node only if
        // this is an odd level node
        if (l % 2 != 0) {
            node.data = arr[index.index];
            (index.index)++;
        }

        // Update nodes in right subtree
        modifyTree(node.right, arr, index, l + 1);
    }

    // A utility function to reverse an array from index
    // 0 to n-1
    void reverse(char arr[], int n) {
        int l = 0, r = n - 1;
        while (l < r) {
            char temp = arr[l];
            arr[l] = arr[r];
            arr[r] = temp;
            l++;
            r--;
        }
    }

    void reverseAlternate()
    {
        reverseAlternate(root);
    }

    // The main function to reverse
    // alternate nodes of a binary tree
    void reverseAlternate(Node node)
    {

        // Create an auxiliary array to store
        // nodes of alternate levels
        char[] arr = new char[100];

        // First store nodes of alternate levels
        storeAlternate(node, arr, index_obj, 0);

        //index_obj.index = 0;

        // Reverse the array
        reverse(arr, index_obj.index);

        // Update tree by taking elements from array
        index_obj.index = 0;
        modifyTree(node, arr, index_obj, 0);
    }

    void printInorder() {
        printInorder(root);
    }

    // A utility function to print
    // indorder traversal of a
    // binary tree
    void printInorder(Node node) {
        if (node == null) {
            return;
        }
        printInorder(node.left);
        System.out.print(node.data + " ");
        printInorder(node.right);
    }

    // Driver program to test the above functions
    public static void main(String args[]) {
        BinaryTree tree = new BinaryTree();
        tree.root = new Node('a');
        tree.root.left = new Node('b');
        tree.root.right = new Node('c');
        tree.root.left.left = new Node('d');
        tree.root.left.right = new Node('e');
        tree.root.right.left = new Node('f');
        tree.root.right.right = new Node('g');
        tree.root.left.left.left = new Node('h');
        tree.root.left.left.right = new Node('i');
        tree.root.left.right.left = new Node('j');
        tree.root.left.right.right = new Node('k');
        tree.root.right.left.left = new Node('l');
        tree.root.right.left.right = new Node('m');
        tree.root.right.right.left = new Node('n');
        tree.root.right.right.right = new Node('o');
        System.out.println("Inorder Traversal of given tree");
        tree.printInorder();

        tree.reverseAlternate();
        System.out.println("");
        System.out.println("");
        System.out.println("Inorder Traversal of modified tree");
        tree.printInorder();
    }
}

// This code has been contributed by Mayank Jaiswal

Python3

# Python3 program to reverse
# alternate levels of a binary tree
MAX = 100

# A Binary Tree node
class Node:

    def __init__(self, data):

        self.left = None
        self.right = None
        self.data = data

# A utility function to
# create a new Binary Tree
# Node
def newNode(item):

    temp = Node(item)
    return temp

# Function to store nodes of
# alternate levels in an array
def storeAlternate(root, arr,
                   index, l):

    # Base case
    if (root == None):
        return index;

    # Store elements of
    # left subtree
    index = storeAlternate(root.left,
                           arr, index,
                           l + 1);

    # Store this node only if
    # this is a odd level node
    if(l % 2 != 0):
        arr[index] = root.data;
        index += 1;

    # Store elements of right
    # subtree
    index=storeAlternate(root.right,
                         arr, index,
                         l + 1);
    return index

# Function to modify Binary Tree
# (All odd level nodes are
# updated by taking elements from
# array in inorder fashion)
def modifyTree(root, arr, index, l):

    # Base case
    if (root == None):
        return index;

    # Update nodes in left subtree
    index=modifyTree(root.left,
                     arr, index,
                     l + 1);

    # Update this node only
    # if this is an odd level
    # node
    if (l % 2 != 0):
        root.data = arr[index];
        index += 1;

    # Update nodes in right
    # subtree
    index=modifyTree(root.right,
                     arr, index,
                     l + 1);
    return index

# A utility function to
# reverse an array from
# index 0 to n-1
def reverse(arr, n):

    l = 0
    r = n - 1;

    while (l < r):
        arr[l], arr[r] = (arr[r],
                          arr[l]);
        l += 1
        r -= 1

# The main function to reverse
# alternate nodes of a binary tree
def reverseAlternate(root):

    # Create an auxiliary array
    # to store nodes of alternate
    # levels
    arr = [0 for i in range(MAX)]
    index = 0;

    # First store nodes of
    # alternate levels
    index=storeAlternate(root, arr,
                         index, 0);

    # Reverse the array
    reverse(arr, index);

    # Update tree by taking
    # elements from array
    index = 0;
    index=modifyTree(root, arr,
                     index, 0);

# A utility function to print
# indorder traversal of a
# binary tree
def printInorder(root):

    if(root == None):
        return;
    printInorder(root.left);
    print(root.data, end = ' ')
    printInorder(root.right);

# Driver code
if __name__=="__main__":

    root = newNode('a');
    root.left = newNode('b');
    root.right = newNode('c');
    root.left.left = newNode('d');
    root.left.right = newNode('e');
    root.right.left = newNode('f');
    root.right.right = newNode('g');
    root.left.left.left = newNode('h');
    root.left.left.right = newNode('i');
    root.left.right.left = newNode('j');
    root.left.right.right = newNode('k');
    root.right.left.left = newNode('l');
    root.right.left.right = newNode('m');
    root.right.right.left = newNode('n');
    root.right.right.right = newNode('o');

    print("Inorder Traversal of given tree")
    printInorder(root);

    reverseAlternate(root);

    print("\nInorder Traversal of modified tree")
    printInorder(root);

# This code is contributed by Rutvik_56

C#

// C# program to reverse alternate
// levels of perfect binary tree
using System;

// A binary tree node
public class Node
{
    public char data;
    public Node left, right;

    public Node(char item)
    {
        data = item;
        left = right = null;
    }
}

// class to access index value
// by reference
public class Index
{
    public int index;
}

class GFG
{
public Node root;
public Index index_obj = new Index();

// function to store alternate
// levels in a tree
public virtual void storeAlternate(Node node, char[] arr,
                                   Index index, int l)
{
    // base case
    if (node == null)
    {
        return;
    }

    // store elements of left subtree
    storeAlternate(node.left, arr, index, l + 1);

    // store this node only if level is odd
    if (l % 2 != 0)
    {
        arr[index.index] = node.data;
        index.index++;
    }

    storeAlternate(node.right, arr, index, l + 1);
}

// Function to modify Binary Tree (All odd
// level nodes are updated by taking elements
// from array in inorder fashion)
public virtual void modifyTree(Node node, char[] arr,
                               Index index, int l)
{

    // Base case
    if (node == null)
    {
        return;
    }

    // Update nodes in left subtree
    modifyTree(node.left, arr, index, l + 1);

    // Update this node only if this
    // is an odd level node
    if (l % 2 != 0)
    {
        node.data = arr[index.index];
        (index.index)++;
    }

    // Update nodes in right subtree
    modifyTree(node.right, arr, index, l + 1);
}

// A utility function to reverse an
// array from index 0 to n-1
public virtual void reverse(char[] arr, int n)
{
    int l = 0, r = n - 1;
    while (l < r)
    {
        char temp = arr[l];
        arr[l] = arr[r];
        arr[r] = temp;
        l++;
        r--;
    }
}

public virtual void reverseAlternate()
{
    reverseAlternate(root);
}

// The main function to reverse
// alternate nodes of a binary tree
public virtual void reverseAlternate(Node node)
{

    // Create an auxiliary array to
    // store nodes of alternate levels
    char[] arr = new char[100];

    // First store nodes of alternate levels
    storeAlternate(node, arr, index_obj, 0);

    //index_obj.index = 0;

    // Reverse the array
    reverse(arr, index_obj.index);

    // Update tree by taking elements from array
    index_obj.index = 0;
    modifyTree(node, arr, index_obj, 0);
}

public virtual void printInorder()
{
    printInorder(root);
}

// A utility function to print indorder
// traversal of a binary tree
public virtual void printInorder(Node node)
{
    if (node == null)
    {
        return;
    }
    printInorder(node.left);
    Console.Write(node.data + " ");
    printInorder(node.right);
}

// Driver Code
public static void Main(string[] args)
{
    GFG tree = new GFG();
    tree.root = new Node('a');
    tree.root.left = new Node('b');
    tree.root.right = new Node('c');
    tree.root.left.left = new Node('d');
    tree.root.left.right = new Node('e');
    tree.root.right.left = new Node('f');
    tree.root.right.right = new Node('g');
    tree.root.left.left.left = new Node('h');
    tree.root.left.left.right = new Node('i');
    tree.root.left.right.left = new Node('j');
    tree.root.left.right.right = new Node('k');
    tree.root.right.left.left = new Node('l');
    tree.root.right.left.right = new Node('m');
    tree.root.right.right.left = new Node('n');
    tree.root.right.right.right = new Node('o');
    Console.WriteLine("Inorder Traversal of given tree");
    tree.printInorder();

    tree.reverseAlternate();
    Console.WriteLine("");
    Console.WriteLine("");
    Console.WriteLine("Inorder Traversal of modified tree");
    tree.printInorder();
}
}

// This code is contributed by Shrikant13

Javascript

<script>
// javascript program to reverse alternate
// levels of  perfect binary tree
// A binary tree node
 class Node {
        constructor(val) {
            this.data = val;
            this.left = null;
            this.right = null;
        }
    }

// class to access index value by reference
 var index = 0;



    // function to store alternate levels in a tree
    function storeAlternate(node,  arr , l) {
        // base case
        if (node == null) {
            return;
        }
        // store elements of left subtree
        storeAlternate(node.left, arr, l + 1);

        // store this node only if level is odd
        if (l % 2 != 0) {
            arr[index] = node.data;
            index++;
        }

        storeAlternate(node.right, arr, l + 1);
    }

    // Function to modify Binary Tree
    // (All odd level nodes are
    // updated by taking elements from
    // array in inorder fashion)
    function modifyTree(node,  arr , l) {

        // Base case
        if (node == null) {
            return;
        }

        // Update nodes in left subtree
        modifyTree(node.left, arr, l + 1);

        // Update this node only if
        // this is an odd level node
        if (l % 2 != 0) {
            node.data = arr[index];
            (index)++;
        }

        // Update nodes in right subtree
        modifyTree(node.right, arr, l + 1);
    }

    // A utility function to reverse an array from index
    // 0 to n-1
    function reverse( arr , n) {
        var l = 0, r = n - 1;
        while (l < r) {
            var temp = arr[l];
            arr[l] = arr[r];
            arr[r] = temp;
            l++;
            r--;
        }
    }


    // The main function to reverse
    // alternate nodes of a binary tree
    function reverseAlternate(node) {

        // Create an auxiliary array to store
        // nodes of alternate levels
        var arr = Array(100).fill('');

        // First store nodes of alternate levels
        storeAlternate(node, arr, 0);

         //index_obj.index = 0;

        // Reverse the array
        reverse(arr, index);

        // Update tree by taking elements from array
        index = 0;
        modifyTree(node, arr, 0);
    }



    // A utility function to print
    // indorder traversal of a
    // binary tree
    function printInorder(node) {
        if (node == null) {
            return;
        }
        printInorder(node.left);
        document.write(node.data + " ");
        printInorder(node.right);
    }
 function newNode(key) {
var temp = new Node();
        temp.left = temp.right = null;
        temp.data =  key;
        return temp;
    }
    // Driver program to test the above functions

        var root = newNode('a');
        root.left = newNode('b');
        root.right = newNode('c');
        root.left.left = newNode('d');
        root.left.right = newNode('e');
        root.right.left = newNode('f');
        root.right.right = newNode('g');
        root.left.left.left = newNode('h');
        root.left.left.right = newNode('i');
        root.left.right.left = newNode('j');
        root.left.right.right = newNode('k');
        root.right.left.left = newNode('l');
        root.right.left.right = newNode('m');
        root.right.right.left = newNode('n');
        root.right.right.right = newNode('o');
        document.write("Inorder Traversal of given tree<br/>");
        printInorder(root);

        reverseAlternate(root);
        document.write("<br/>");
        document.write("<br/>");
        document.write("Inorder Traversal of modified tree<br/>");
        printInorder(root);

// This code is contributed by gauravrajput1
</script>

Output

Inorder Traversal of given tree
h d i b j e k a l f m c n g o 

Inorder Traversal of modified tree
o d n c m e l a k f j b i g h 

Time Complexity: O(n), As it does two inorder traversals of the binary tree.
Auxiliary Space: O(n), The extra space is used to store the odd level nodes of the tree and in recursive function call stack which is O(h), where h is the height of the tree.

Method 3 (Using One Traversal):

This method simply swaps the values of the children node, if the current node is on an even level. Because that ultimately swaps elements on an odd level.

i.e for given example:  We discover node a, on level 0, we swap values of left and right node of a.
Result: Level 1(odd) elements get swapped.

Now the tree becomes:

      a
    /   \
   c     b
  / \   / \
 ... ... ...

Which is our desired result of 1st recursion. Hence, we further call same recursive function for child elements.

For the Stack of recursion, as this is a perfect Binary tree, it might have been O(N) for a normal Binary tree

C++

// C++ program to reverse
// alternate levels of a tree
#include <bits/stdc++.h>
using namespace std;

struct Node
{
    char key;
    Node *left, *right;
};

void preorder(struct Node *root1, struct Node*
                               root2, int lvl)
{
    // Base cases
    if (root1 == NULL || root2==NULL)
        return;

    // Swap subtrees if level is even
    if (lvl%2 == 0)
        swap(root1->key, root2->key);

    // Recur for left and right
    // subtrees (Note : left of root1
    // is passed and right of root2 in
    // first call and opposite
    // in second call.
    preorder(root1->left, root2->right, lvl+1);
    preorder(root1->right, root2->left, lvl+1);
}

// This function calls preorder()
// for left and right children
// of root
void reverseAlternate(struct Node *root)
{
   preorder(root->left, root->right, 0);
}

// Inorder traversal (used to print initial and
// modified trees)
void printInorder(struct Node *root)
{
    if (root == NULL)
       return;
    printInorder(root->left);
    cout << root->key << " ";
    printInorder(root->right);
}

// A utility function to create a new node
Node *newNode(int key)
{
    Node *temp = new Node;
    temp->left = temp->right = NULL;
    temp->key = key;
    return temp;
}

// Driver program to test above functions
int main()
{
    struct Node *root = newNode('a');
    root->left = newNode('b');
    root->right = newNode('c');
    root->left->left = newNode('d');
    root->left->right = newNode('e');
    root->right->left = newNode('f');
    root->right->right = newNode('g');
    root->left->left->left = newNode('h');
    root->left->left->right = newNode('i');
    root->left->right->left = newNode('j');
    root->left->right->right = newNode('k');
    root->right->left->left = newNode('l');
    root->right->left->right = newNode('m');
    root->right->right->left = newNode('n');
    root->right->right->right = newNode('o');

    cout << "Inorder Traversal of given tree\n";
    printInorder(root);

    reverseAlternate(root);

    cout << "\n\nInorder Traversal of modified tree\n";
    printInorder(root);
    return 0;
}

Java

// Java program to reverse
// alternate levels of a tree
class Sol
{

    static class Node
    {
        char key;
        Node left, right;
    };

    static void preorder(Node root1,
                       Node root2, int lvl)
    {
        // Base cases
        if (root1 == null || root2 == null)
            return;

        // Swap subtrees if level is even
        if (lvl % 2 == 0) {
            char t = root1.key;
            root1.key = root2.key;
            root2.key = t;
        }

        // Recur for left and right subtrees
        // (Note : left of root1
        // is passed and right of root2 in first
        // call and opposite
        // in second call.
        preorder(root1.left, root2.right,
                                  lvl + 1);
        preorder(root1.right, root2.left,
                                    lvl + 1);
    }

    // This function calls preorder()
    // for left and right
    // children of root
    static void reverseAlternate(Node root)
    {
        preorder(root.left, root.right, 0);
    }

    // Inorder traversal (used to
    // print initial and
    // modified trees)
    static void printInorder(Node root)
    {
        if (root == null)
            return;
        printInorder(root.left);
        System.out.print(root.key + " ");
        printInorder(root.right);
    }

    // A utility function to create a new node
    static Node newNode(int key)
    {
        Node temp = new Node();
        temp.left = temp.right = null;
        temp.key = (char)key;
        return temp;
    }

    // Driver program to test above functions
    public static void main(String args[])
    {
        Node root = newNode('a');
        root.left = newNode('b');
        root.right = newNode('c');
        root.left.left = newNode('d');
        root.left.right = newNode('e');
        root.right.left = newNode('f');
        root.right.right = newNode('g');
        root.left.left.left = newNode('h');
        root.left.left.right = newNode('i');
        root.left.right.left = newNode('j');
        root.left.right.right = newNode('k');
        root.right.left.left = newNode('l');
        root.right.left.right = newNode('m');
        root.right.right.left = newNode('n');
        root.right.right.right = newNode('o');

        System.out.print(
            "Inorder Traversal of given tree\n");
        printInorder(root);

        reverseAlternate(root);

        System.out.print(
            "\n\nInorder Traversal of modified tree\n");
        printInorder(root);
    }
}

// This code is contributed by Arnab Kundu

Python3

# Python3 program to reverse
# alternate levels of a tree

# A Binary Tree Node
# Utility function to create
# a new tree node
class Node:

    # Constructor to create a new node
    def __init__(self, key):
        self.key = key
        self.left = None
        self.right = None

def preorder(root1, root2, lvl):

    # Base cases
    if (root1 == None or root2 == None):
        return

    # Swap subtrees if level is even
    if (lvl % 2 == 0):
        t = root1.key
        root1.key = root2.key
        root2.key = t

    # Recur for left and right subtrees
    # (Note : left of root1 is passed and
    # right of root2 in first call and
    # opposite in second call.
    preorder(root1.left, root2.right, lvl + 1)
    preorder(root1.right, root2.left, lvl + 1)

# This function calls preorder()
# for left and right children of root
def reverseAlternate(root):
    preorder(root.left, root.right, 0)

# Inorder traversal (used to print
# initial and modified trees)
def printInorder(root):
    if (root == None):
        return
    printInorder(root.left)
    print( root.key, end = " ")
    printInorder(root.right)

# A utility function to create a new node
def newNode(key):
    temp = Node(' ')
    temp.left = temp.right = None
    temp.key = key
    return temp

# Driver Code
if __name__ == '__main__':
    root = newNode('a')
    root.left = newNode('b')
    root.right = newNode('c')
    root.left.left = newNode('d')
    root.left.right = newNode('e')
    root.right.left = newNode('f')
    root.right.right = newNode('g')
    root.left.left.left = newNode('h')
    root.left.left.right = newNode('i')
    root.left.right.left = newNode('j')
    root.left.right.right = newNode('k')
    root.right.left.left = newNode('l')
    root.right.left.right = newNode('m')
    root.right.right.left = newNode('n')
    root.right.right.right = newNode('o')

    print( "Inorder Traversal of given tree")
    printInorder(root)

    reverseAlternate(root)

    print("\nInorder Traversal of modified tree")
    printInorder(root)

# This code is contributed by Arnab Kundu

C#

// C# program to reverse alternate
// levels of a tree
using System;

class GFG
{

public class Node
{
    public char key;
    public Node left, right;
};

static void preorder( Node root1,
                       Node root2, int lvl)
{
    // Base cases
    if (root1 == null || root2==null)
        return;

    // Swap subtrees if level is even
    if (lvl % 2 == 0)
        {
            char t = root1.key;
            root1.key = root2.key;
            root2.key = t;
        }

    // Recur for left and right subtrees
    // (Note : left of root1
    // is passed and right of root2 in
    // first call and opposite
    // in second call.
    preorder(root1.left, root2.right, lvl+1);
    preorder(root1.right, root2.left, lvl+1);
}

// This function calls preorder() for left
// and right children
// of root
static void reverseAlternate( Node root)
{
    preorder(root.left, root.right, 0);
}

// Inorder traversal (used to print initial and
// modified trees)
static void printInorder( Node root)
{
    if (root == null)
        return;
    printInorder(root.left);
    Console.Write( root.key + " ");
    printInorder(root.right);
}

// A utility function to create a new node
static Node newNode(int key)
{
    Node temp = new Node();
    temp.left = temp.right = null;
    temp.key = (char)key;
    return temp;
}

// Driver code
public static void Main(String []args)
{
    Node root = newNode('a');
    root.left = newNode('b');
    root.right = newNode('c');
    root.left.left = newNode('d');
    root.left.right = newNode('e');
    root.right.left = newNode('f');
    root.right.right = newNode('g');
    root.left.left.left = newNode('h');
    root.left.left.right = newNode('i');
    root.left.right.left = newNode('j');
    root.left.right.right = newNode('k');
    root.right.left.left = newNode('l');
    root.right.left.right = newNode('m');
    root.right.right.left = newNode('n');
    root.right.right.right = newNode('o');

    Console.Write("Inorder Traversal of given tree\n");
    printInorder(root);

    reverseAlternate(root);

    Console.Write("\n\nInorder Traversal of modified tree\n");
    printInorder(root);

}
}

// This code is contributed by Princi Singh

Javascript

<script>
// javascript program to reverse
// alternate levels of a tree
     class Node {
            constructor(val) {
                this.key = val;
                this.left = null;
                this.right = null;
            }
        }
    function preorder(root1,  root2 , lvl)
    {

        // Base cases
        if (root1 == null || root2 == null)
            return;

        // Swap subtrees if level is even
        if (lvl % 2 == 0) {
            var t = root1.key;
            root1.key = root2.key;
            root2.key = t;
        }

        // Recur for left and right subtrees
        // (Note : left of root1
        // is passed and right of root2 in first
        // call and opposite
        // in second call.
        preorder(root1.left, root2.right, lvl + 1);
        preorder(root1.right, root2.left, lvl + 1);
    }

    // This function calls preorder()
    // for left and right
    // children of root
    function reverseAlternate(root) {
        preorder(root.left, root.right, 0);
    }

    // Inorder traversal (used to
    // print initial and
    // modified trees)
    function printInorder(root) {
        if (root == null)
            return;
        printInorder(root.left);
        document.write(root.key + " ");
        printInorder(root.right);
    }

    // A utility function to create a new node
    function newNode(key) {
var temp = new Node();
        temp.left = temp.right = null;
        temp.key =  key;
        return temp;
    }

    // Driver program to test above functions

var root = newNode('a');
        root.left = newNode('b');
        root.right = newNode('c');
        root.left.left = newNode('d');
        root.left.right = newNode('e');
        root.right.left = newNode('f');
        root.right.right = newNode('g');
        root.left.left.left = newNode('h');
        root.left.left.right = newNode('i');
        root.left.right.left = newNode('j');
        root.left.right.right = newNode('k');
        root.right.left.left = newNode('l');
        root.right.left.right = newNode('m');
        root.right.right.left = newNode('n');
        root.right.right.right = newNode('o');

        document.write("Inorder Traversal of given tree<br\>");
        printInorder(root);

        reverseAlternate(root);

        document.write("<br\><br\>Inorder Traversal of modified tree<br\>");
        printInorder(root);

// This code is contributed by umadevi9616
</script>

Output

Inorder Traversal of given tree
h d i b j e k a l f m c n g o 

Inorder Traversal of modified tree
o d n c m e l a k f j b i g h 

Time Complexity: O(N)
Auxiliary Space: O(log N), this is due to the recursive call stack.