Flatten Binary Tree in order of Zig Zag traversal

Given a Binary Tree, the task is to flatten it in order of ZigZag traversal of the tree. In the flattened binary tree, the left node of all the nodes must be NULL.
Examples:

Input: 
          1 
        /   \ 
       5     2 
      / \   / \ 
     6   4 9   3 
Output: 1 2 5 6 4 9 3

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

Approach: We will solve this problem by simulating the ZigZag traversal of Binary Tree. 
Algorithm: 

1. Create two stacks, “c_lev” and “n_lev” and to store the nodes of current and next level Binary tree.
2. Create a variable “prev” and initialise it by parent node.
3. Push right and left children of parent in the c_lev stack.
4. Apply ZigZag traversal. Lets say “curr” is top most element in “c_lev”. Then, 
   • If ‘curr’ is NULL, continue.
   • Else push curr->left and curr->right on the stack “n_lev” in appropriate order. If we are performing left to right traversal then curr->left is pushed first else curr->right is pushed first.
   • Set prev = curr.

Below is the implementation of the above approach:

CPP

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;

// Node of the binary tree
struct node {
    int data;
    node* left;
    node* right;
    node(int data)
    {
        this->data = data;
        left = NULL;
        right = NULL;
    }
};

// Function to flatten Binary tree
void flatten(node* parent)
{
    // Queue to store node
    // for BFS
    stack<node *> c_lev, n_lev;

    c_lev.push(parent->left);
    c_lev.push(parent->right);

    bool lev = 1;
    node* prev = parent;

    // Code for BFS
    while (c_lev.size()) {

        // Size of queue
        while (c_lev.size()) {

            // Front most node in
            // queue
            node* curr = c_lev.top();
            c_lev.pop();

            // Base case
            if (curr == NULL)
                continue;
            prev->right = curr;
            prev->left = NULL;
            prev = curr;

            // Pushing new elements
            // in queue
            if (!lev)
                n_lev.push(curr->left);
            n_lev.push(curr->right);
            if (lev)
                n_lev.push(curr->left);
        }
        lev = (!lev);
        c_lev = n_lev;
        while (n_lev.size())
            n_lev.pop();
    }

    prev->left = NULL;
    prev->right = NULL;
}

// Function to print flattened
// binary tree
void print(node* parent)
{
    node* curr = parent;
    while (curr != NULL)
        cout << curr->data << " ", curr = curr->right;
}

// Driver code
int main()
{
    node* root = new node(1);
    root->left = new node(5);
    root->right = new node(2);
    root->left->left = new node(6);
    root->left->right = new node(4);
    root->right->left = new node(9);
    root->right->right = new node(3);

    // Calling required functions
    flatten(root);

    print(root);

    return 0;
}

Javascript

<script>

// Javascript implementation of the approach

// Node of the binary tree
class node {

    constructor(data)
    {
        this.data = data;
        this.left = null;
        this.right = null;
    }
};

// Function to flatten Binary tree
function flatten()
{
    // Queue to store node
    // for BFS
    var c_lev = [], n_lev = [];

    c_lev.push(parent.left);
    c_lev.push(parent.right);

    var lev = 1;
    var prev = parent;

    // Code for BFS
    while (c_lev.length!=0) {

        // Size of queue
        while (c_lev.length!=0) {

            // Front most node in
            // queue
            var curr = c_lev[c_lev.length-1];
            c_lev.pop();

            // Base case
            if (curr == null)
                continue;

            prev.right = curr;
            prev.left = null;
            prev = curr;

            // Pushing new elements
            // in queue
            if (!lev)
                n_lev.push(curr.left);
            n_lev.push(curr.right);
            if (lev)
                n_lev.push(curr.left);
        }

        lev = lev==0?1:0;
        c_lev = n_lev;
        n_lev = [];
    }

    prev.left = null;
    prev.right = null;
}

// Function to print flattened
// binary tree
function print()
{
    var curr = parent;
    while (curr != null)
    {
        document.write(curr.data + " ");
        curr = curr.right;
    }
}

// Driver code
var parent = new node(1);
parent.left = new node(5);
parent.right = new node(2);
parent.left.left = new node(6);
parent.left.right = new node(4);
parent.right.left = new node(9);
parent.right.right = new node(3);
// Calling required functions
flatten();
print();

// This code is contributed by rrrtnx.
</script>

Output:

1 2 5 6 4 9 3

Time Complexity: O(N) 
Space Complexity: O(N) where N is the size of Binary Tree.