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# Create a Doubly Linked List from a Ternary Tree

• Difficulty Level : Medium
• Last Updated : 16 Jun, 2021

Given a ternary tree, create a doubly linked list out of it. A ternary tree is just like binary tree but instead of having two nodes, it has three nodes i.e. left, middle, right.

The doubly linked list should holds following properties –

1. Left pointer of ternary tree should act as prev pointer of doubly linked list.
2. Middle pointer of ternary tree should not point to anything.
3. Right pointer of ternary tree should act as next pointer of doubly linked list.
4. Each node of ternary tree is inserted into doubly linked list before its subtrees and for any node, its left child will be inserted first, followed by mid and right child (if any).

For the above example, the linked list formed for below tree should be NULL <- 30 <-> 5 <-> 1 <-> 4 <-> 8 <-> 11 <-> 6 <-> 7 <-> 15 <-> 63 <-> 31 <-> 55 <-> 65 -> NULL We strongly recommend you to minimize your browser and try this yourself first.
The idea is to traverse the tree in preoder fashion similar to binary tree preorder traversal. Here, when we visit a node, we will insert it into doubly linked list in the end using a tail pointer. That we use to maintain the required insertion order. We then recursively call for left child, middle child and right child in that order.

Below is the implementation of this idea.

## C++

 `// C++ program to create a doubly linked list out``// of given a ternary tree.``#include ``using` `namespace` `std;` `/* A ternary tree */``struct` `Node``{``    ``int` `data;``    ``struct` `Node *left, *middle, *right;``};` `/* Helper function that allocates a new node with the``   ``given data and assign NULL to left, middle and right``   ``pointers.*/``Node* newNode(``int` `data)``{``    ``Node* node = ``new` `Node;``    ``node->data = data;``    ``node->left = node->middle = node->right = NULL;``    ``return` `node;``}` `/* Utility function that constructs doubly linked list``by inserting current node at the end of the doubly``linked list by using a tail pointer */``void` `push(Node** tail_ref, Node* node)``{``    ``// initialize tail pointer``    ``if` `(*tail_ref == NULL)``    ``{``        ``*tail_ref = node;` `        ``// set left, middle and right child to point``        ``// to NULL``        ``node->left = node->middle = node->right = NULL;` `        ``return``;``    ``}` `    ``// insert node in the end using tail pointer``    ``(*tail_ref)->right = node;` `    ``// set prev of node``    ``node->left = (*tail_ref);` `    ``// set middle and right child to point to NULL``    ``node->right = node->middle = NULL;` `    ``// now tail pointer will point to inserted node``    ``(*tail_ref) = node;``}` `/* Create a doubly linked list out of given a ternary tree.``by traversing the tree in preoder fashion. */``Node* TernaryTreeToList(Node* root, Node** head_ref)``{``    ``// Base case``    ``if` `(root == NULL)``        ``return` `NULL;` `    ``//create a static tail pointer``    ``static` `Node* tail = NULL;` `    ``// store left, middle and right nodes``    ``// for future calls.``    ``Node* left = root->left;``    ``Node* middle = root->middle;``    ``Node* right = root->right;` `    ``// set head of the doubly linked list``    ``// head will be root of the ternary tree``    ``if` `(*head_ref == NULL)``        ``*head_ref = root;` `    ``// push current node in the end of DLL``    ``push(&tail, root);` `    ``//recurse for left, middle and right child``    ``TernaryTreeToList(left, head_ref);``    ``TernaryTreeToList(middle, head_ref);``    ``TernaryTreeToList(right, head_ref);``}` `// Utility function for printing double linked list.``void` `printList(Node* head)``{``    ``printf``(``"Created Double Linked list is:\n"``);``    ``while` `(head)``    ``{``        ``printf``(``"%d "``, head->data);``        ``head = head->right;``    ``}``}` `// Driver program to test above functions``int` `main()``{``    ``// Construting ternary tree as shown in above figure``    ``Node* root = newNode(30);` `    ``root->left = newNode(5);``    ``root->middle = newNode(11);``    ``root->right = newNode(63);` `    ``root->left->left = newNode(1);``    ``root->left->middle = newNode(4);``    ``root->left->right = newNode(8);` `    ``root->middle->left = newNode(6);``    ``root->middle->middle = newNode(7);``    ``root->middle->right = newNode(15);` `    ``root->right->left = newNode(31);``    ``root->right->middle = newNode(55);``    ``root->right->right = newNode(65);` `    ``Node* head = NULL;` `    ``TernaryTreeToList(root, &head);` `    ``printList(head);` `    ``return` `0;``}`

## Java

 `//Java program to create a doubly linked list``// from a given ternary tree.` `//Custom node class.``class` `newNode``{``    ``int` `data;``    ``newNode left,middle,right;``    ``public` `newNode(``int` `data)``    ``{``        ``this``.data = data;``        ``left = middle = right = ``null``;``    ``}``}` `class` `GFG {``    ` `    ``//tail of the linked list.``    ``static` `newNode tail;` `    ``//function to push the node to the tail.``    ``public` `static` `void` `push(newNode node)``    ``{``        ``//to put the node at the end of``        ``// the already existing tail.``        ``tail.right = node;                ``        ` `        ``//to point to the previous node.``        ``node.left = tail;        ``        ` `        ``// middle pointer should point to``        ``// nothing so null. initiate right``        ``// pointer to null.``        ``node.middle = node.right = ``null``;``        ` `        ``//update the tail position.``        ``tail = node;            ``    ``}``    ` `    ``/* Create a doubly linked list out of given a ternary tree.``    ``by traversing the tree in preoder fashion. */``    ``public` `static` `void` `ternaryTree(newNode node,newNode head)``    ``{``        ``if``(node == ``null``)``            ``return``;                    ``        ``newNode left = node.left;``        ``newNode middle = node.middle;``        ``newNode right = node.right;``        ``if``(tail != node)``        ` `            ``// already root is in the tail so dont push``            ``// the node when it was root.In the first``            ``// case both node and tail have root in them.``            ``push(node);            ``            ` `        ``// First the left child is to be taken.``        ``// Then middle and then right child.``        ``ternaryTree(left,head);        ``        ``ternaryTree(middle,head);``        ``ternaryTree(right,head);``    ``}` `    ``//function to initiate the list process.``    ``public` `static` `newNode startTree(newNode root)``    ``{``        ``//Initiate the head and tail with root.``        ``newNode head = root;``        ``tail = root;``        ``ternaryTree(root,head);``        ` `        ``//since the head,root are passed``        ``// with reference the changes in``        ``// root will be reflected in head.``        ``return` `head;        ``    ``}``    ` `    ``// Utility function for printing double linked list.``    ``public` `static` `void` `printList(newNode head)``    ``{``        ``System.out.print(``"Created Double Linked list is:\n"``);``        ``while``(head != ``null``)``        ``{``            ``System.out.print(head.data + ``" "``);``            ``head = head.right;``        ``}``    ``}``    ` `    ``// Driver program to test above functions``    ``public` `static` `void` `main(String args[])``    ``{``        ` `        ``// Construting ternary tree as shown``        ``// in above figure``        ``newNode root = ``new` `newNode(``30``);``        ``root.left = ``new` `newNode(``5``);``        ``root.middle = ``new` `newNode(``11``);``        ``root.right = ``new` `newNode(``63``);``        ``root.left.left = ``new` `newNode(``1``);``        ``root.left.middle = ``new` `newNode(``4``);``        ``root.left.right = ``new` `newNode(``8``);``        ``root.middle.left = ``new` `newNode(``6``);``        ``root.middle.middle = ``new` `newNode(``7``);``        ``root.middle.right = ``new` `newNode(``15``);``        ``root.right.left = ``new` `newNode(``31``);``        ``root.right.middle = ``new` `newNode(``55``);``        ``root.right.right = ``new` `newNode(``65``);``        ` `        ``// The function which initiates the list``        ``// process returns the head.``        ``newNode head = startTree(root);        ``        ``printList(head);``    ``}``}` `// This code is contributed by M.V.S.Surya Teja.`

## Python3

 `# Python3 program to create a doubly linked``# list out of given a ternary tree.``  ` `# Custom node class.``class` `newNode:``    ` `    ``def` `__init__(``self``, data):``        ` `        ``self``.data ``=` `data``        ``self``.left ``=` `None``        ``self``.right ``=` `None``        ``self``.middle ``=` `None` `class` `GFG:``    ` `    ``def` `__init__(``self``):``        ` `        ``# Tail of the linked list.``        ``self``.tail ``=` `None` `    ``# Function to push the node to the tail.``    ``def` `push(``self``, node):` `        ``# To put the node at the end of``        ``# the already existing tail.``        ``self``.tail.right ``=` `node` `        ``# To point to the previous node.``        ``node.left ``=` `self``.tail` `        ``# Middle pointer should point to ``        ``# nothing so null. initiate right``        ``# pointer to null.``        ``node.middle ``=` `node.right ``=` `None` `        ``# Update the tail position.``        ``self``.tail ``=` `node` `    ``# Create a doubly linked list out of given``    ``# a ternary tree By traversing the tree in``    ``# preoder fashion.``    ``def` `ternaryTree(``self``, node, head):``        ` `        ``if` `node ``=``=` `None``:``            ``return` `        ``left ``=` `node.left``        ``middle ``=` `node.middle``        ``right ``=` `node.right``        ` `        ``if` `self``.tail !``=` `node:``            ` `            ``# Already root is in the tail so dont push ``            ``# the node when it was root.In the first ``            ``# case both node and tail have root in them.``            ``self``.push(node)` `        ``# First the left child is to be taken.``        ``# Then middle and then right child.``        ``self``.ternaryTree(left, head) ``        ``self``.ternaryTree(middle, head)``        ``self``.ternaryTree(right, head)` `    ``def` `startTree(``self``, root):``        ` `        ``# Initiate the head and tail with root.``        ``head ``=` `root``        ``self``.tail ``=` `root``        ``self``.ternaryTree(root, head)` `        ``# Since the head,root are passed ``        ``# with reference the changes in ``        ``# root will be reflected in head.``        ``return` `head` `    ``# Utility function for printing double linked list.``    ``def` `printList(``self``, head):``        ` `        ``print``(``"Created Double Linked list is:"``)``        ` `        ``while` `head:``            ``print``(head.data, end ``=` `" "``)``            ``head ``=` `head.right` `# Driver code``if` `__name__ ``=``=` `'__main__'``:``    ` `    ``# Construting ternary tree as shown``    ``# in above figure``    ``root ``=` `newNode(``30``)``    ``root.left ``=` `newNode(``5``)``    ``root.middle ``=` `newNode(``11``)``    ``root.right ``=` `newNode(``63``)``    ``root.left.left ``=` `newNode(``1``)``    ``root.left.middle ``=` `newNode(``4``)``    ``root.left.right ``=` `newNode(``8``)``    ``root.middle.left ``=` `newNode(``6``)``    ``root.middle.middle ``=` `newNode(``7``)``    ``root.middle.right ``=` `newNode(``15``)``    ``root.right.left ``=` `newNode(``31``)``    ``root.right.middle ``=` `newNode(``55``)``    ``root.right.right ``=` `newNode(``65``)` `    ``# The function which initiates the list ``    ``# process returns the head.``    ``head ``=` `None``    ``gfg ``=` `GFG()``    ``head ``=` `gfg.startTree(root)``    ` `    ``gfg.printList(head)` `# This code is contributed by Winston Sebastian Pais`

## C#

 `// C# program to create a doubly linked``// list from a given ternary tree.``using` `System;` `// Custom node class.``public` `class` `newNode``{``    ``public` `int` `data;``    ``public` `newNode left, middle, right;``    ``public` `newNode(``int` `data)``    ``{``        ``this``.data = data;``        ``left = middle = right = ``null``;``    ``}``}` `class` `GFG``{` `// tail of the linked list.``public` `static` `newNode tail;` `// function to push the node to the tail.``public` `static` `void` `push(newNode node)``{``    ``// to put the node at the end of``    ``// the already existing tail.``    ``tail.right = node;` `    ``// to point to the previous node.``    ``node.left = tail;` `    ``// middle pointer should point to``    ``// nothing so null. initiate right``    ``// pointer to null.``    ``node.middle = node.right = ``null``;` `    ``// update the tail position.``    ``tail = node;``}` `/* Create a doubly linked list out``of given a ternary tree. by traversing``the tree in preoder fashion. */``public` `static` `void` `ternaryTree(newNode node,``                               ``newNode head)``{``    ``if` `(node == ``null``)``    ``{``        ``return``;``    ``}``    ``newNode left = node.left;``    ``newNode middle = node.middle;``    ``newNode right = node.right;``    ``if` `(tail != node)``    ``{` `        ``// already root is in the tail so dont push``        ``// the node when it was root.In the first``        ``// case both node and tail have root in them.``        ``push(node);``    ``}` `    ``// First the left child is to be taken.``    ``// Then middle and then right child.``    ``ternaryTree(left, head);``    ``ternaryTree(middle, head);``    ``ternaryTree(right, head);``}` `// function to initiate the list process.``public` `static` `newNode startTree(newNode root)``{``    ``// Initiate the head and tail with root.``    ``newNode head = root;``    ``tail = root;``    ``ternaryTree(root,head);` `    ``// since the head,root are passed``    ``// with reference the changes in``    ``// root will be reflected in head.``    ``return` `head;``}` `// Utility function for printing``// double linked list.``public` `static` `void` `printList(newNode head)``{``    ``Console.Write(``"Created Double Linked list is:\n"``);``    ``while` `(head != ``null``)``    ``{``        ``Console.Write(head.data + ``" "``);``        ``head = head.right;``    ``}``}` `// Driver Code``public` `static` `void` `Main(``string``[] args)``{` `    ``// Construting ternary tree as shown``    ``// in above figure``    ``newNode root = ``new` `newNode(30);``    ``root.left = ``new` `newNode(5);``    ``root.middle = ``new` `newNode(11);``    ``root.right = ``new` `newNode(63);``    ``root.left.left = ``new` `newNode(1);``    ``root.left.middle = ``new` `newNode(4);``    ``root.left.right = ``new` `newNode(8);``    ``root.middle.left = ``new` `newNode(6);``    ``root.middle.middle = ``new` `newNode(7);``    ``root.middle.right = ``new` `newNode(15);``    ``root.right.left = ``new` `newNode(31);``    ``root.right.middle = ``new` `newNode(55);``    ``root.right.right = ``new` `newNode(65);` `    ``// The function which initiates the list``    ``// process returns the head.``    ``newNode head = startTree(root);``    ``printList(head);``}``}` `// This code is contributed by Shrikant13`

## Javascript

 ``

Output:

```Created Double Linked list is:
30 5 1 4 8 11 6 7 15 63 31 55 65```