Create a Doubly Linked List from a Ternary Tree

Last Updated : 01 Jul, 2022

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

The doubly linked list should hold the following properties –

The left pointer of the ternary tree should act as prev pointer of the doubly linked list.
The middle pointer of the ternary tree should not point to anything.
Right pointer of the ternary tree should act as the next pointer of the doubly linked list.
Each node of the ternary tree is inserted into the doubly linked list before its subtrees and for any node, its left child will be inserted first, followed by the 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

tree

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The idea is to traverse the tree in a preorder fashion similar to binary tree preorder traversal. Here, when we visit a node, we will insert it into a 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 <bits/stdc++.h>
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 preorder fashion. */
void TernaryTreeToList(Node* root, Node** head_ref)
{
    // Base case
    if (root == NULL)
        return;
 
    //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()
{
    // Constructing 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 preorder 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[])
    {
         
        // Constructing 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
    # preorder 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__':
     
    # Constructing 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 preorder 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)
{
 
    // Constructing 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

   <script>
//javascript program to create a doubly linked list 
// from a given ternary tree.
 
//Custom node class.
class newNode {
     
    constructor(data) {
        this.data = data;
        this.left = null;
        this.middle = null;
        this.right = null;
    }
}
    // tail of the linked list.
     var tail;
 
    // function to push the node to the tail.
    function push( 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 preorder fashion.
     */
    function ternaryTree( node,  head) {
        if (node == null)
            return;
        var left = node.left;
        var middle = node.middle;
        var 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.
      function startTree( root) {
        // Initiate the head and tail with root.
        var 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 var linked list.
    function printList( head) {
        document.write("Created Double Linked list is:<br/>");
        while (head != null) {
            document.write(head.data + " ");
            head = head.right;
        }
    }
 
    // Driver program to test above functions
     
 
        // Constructing ternary tree as shown
        // in above figure
         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.
         head = startTree(root);
        printList(head);
 
// This code contributed by gauravrajput1 
</script>
 
  

Output

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

Time Complexity: O(n), as we are using recursion to traverse n times. Where n is the number of nodes in the tree.
Auxiliary Space: O(n), as we are using extra space for the linked list.

Introduction to Binary Search Tree

Aditya Goel

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Create a Doubly Linked List from a Ternary Tree

C++

Java

Python3

C#

Javascript

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