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

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

Algorithm:

- Create two stacks, “c_lev” and “n_lev” and to store the nodes of current and next level Binary tree.
- Create a variable “prev” and initialise it by parent node.
- Push right and left children of parent in the c_lev stack.
- 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:

`// 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;` `}` |

**Output:**

1 2 5 6 4 9 3

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

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