Convert a given Binary Tree to Doubly Linked List | Set 4 - GeeksforGeeks

10 May, 2021
Given a Binary Tree (BT), convert it to a Doubly Linked List(DLL) In-Place. The left and right pointers in nodes are to be used as previous and next pointers respectively in converted DLL. The order of nodes in DLL must be same as Inorder of the given Binary Tree. The first node of Inorder traversal (left most node in BT) must be head node of the DLL. 
 
Below three different solutions have been discussed for this problem. Convert a given Binary Tree to Doubly Linked List | Set 1 Convert a given Binary Tree to Doubly Linked List | Set 2 Convert a given Binary Tree to Doubly Linked List | Set 3In the following implementation, we traverse the tree in inorder fashion. We add nodes at the beginning of current linked list and update head of the list using pointer to head pointer. Since we insert at the beginning, we need to process leaves in reverse order. For reverse order, we first traverse the right subtree before the left subtree. i.e. do a reverse inorder traversal. 
C++
Java
Python3
C#
Javascript
// C++ program to convert a given Binary Tree to Doubly Linked List#include <bits/stdc++.h> // Structure for tree and linked liststruct Node {    int data;    Node *left, *right;}; // Utility function for allocating node for Binary// Tree.Node* newNode(int data){    Node* node = new Node;    node->data = data;    node->left = node->right = NULL;    return node;} // A simple recursive function to convert a given// Binary tree to Doubly Linked List// root    --> Root of Binary Tree// head --> Pointer to head node of created doubly linked listvoid BToDLL(Node* root, Node*& head){    // Base cases    if (root == NULL)        return;     // Recursively convert right subtree    BToDLL(root->right, head);     // insert root into DLL    root->right = head;     // Change left pointer of previous head    if (head != NULL)        head->left = root;     // Change head of Doubly linked list    head = root;     // Recursively convert left subtree    BToDLL(root->left, head);} // Utility function for printing double linked list.void printList(Node* head){    printf("Extracted Double Linked list is:\n");    while (head) {        printf("%d ", head->data);        head = head->right;    }} // Driver program to test above functionint main(){    /* Constructing below tree            5            / \            3     6        / \     \        1 4     8        / \     / \        0 2     7 9 */    Node* root = newNode(5);    root->left = newNode(3);    root->right = newNode(6);    root->left->left = newNode(1);    root->left->right = newNode(4);    root->right->right = newNode(8);    root->left->left->left = newNode(0);    root->left->left->right = newNode(2);    root->right->right->left = newNode(7);    root->right->right->right = newNode(9);     Node* head = NULL;    BToDLL(root, head);     printList(head);     return 0;}
// Java program to convert a given Binary Tree to Doubly Linked List /* Structure for tree and Linked List */class Node {    int data;    Node left, right;     public Node(int data)    {        this.data = data;        left = right = null;    }} class BinaryTree {    // root    --> Root of Binary Tree    Node root;     // head --> Pointer to head node of created doubly linked list    Node head;     // A simple recursive function to convert a given    // Binary tree to Doubly Linked List    void BToDLL(Node root)    {        // Base cases        if (root == null)            return;         // Recursively convert right subtree        BToDLL(root.right);         // insert root into DLL        root.right = head;         // Change left pointer of previous head        if (head != null)            head.left = root;         // Change head of Doubly linked list        head = root;         // Recursively convert left subtree        BToDLL(root.left);    }     // Utility function for printing double linked list.    void printList(Node head)    {        System.out.println("Extracted Double Linked List is : ");        while (head != null) {            System.out.print(head.data + " ");            head = head.right;        }    }     // Driver program to test the above functions    public static void main(String[] args)    {        /* Constructing below tree               5             /   \            3     6           / \     \          1   4     8         / \       / \        0   2     7   9  */         BinaryTree tree = new BinaryTree();        tree.root = new Node(5);        tree.root.left = new Node(3);        tree.root.right = new Node(6);        tree.root.left.right = new Node(4);        tree.root.left.left = new Node(1);        tree.root.right.right = new Node(8);        tree.root.left.left.right = new Node(2);        tree.root.left.left.left = new Node(0);        tree.root.right.right.left = new Node(7);        tree.root.right.right.right = new Node(9);         tree.BToDLL(tree.root);        tree.printList(tree.head);    }} // This code has been contributed by Mayank Jaiswal(mayank_24)
# Python3 program to convert a given Binary Tree to Doubly Linked Listclass Node:    def __init__(self, data):        self.data = data        self.left = self.right = None class BinaryTree:    # A simple recursive function to convert a given    # Binary tree to Doubly Linked List    # root    --> Root of Binary Tree    # head --> Pointer to head node of created doubly linked list    root, head = None, None         def BToDll(self, root: Node):        if root is None:            return         # Recursively convert right subtree        self.BToDll(root.right)         # Insert root into doubly linked list        root.right = self.head         # Change left pointer of previous head        if self.head is not None:            self.head.left = root         # Change head of doubly linked list        self.head = root         # Recursively convert left subtree        self.BToDll(root.left)     @staticmethod    def print_list(head: Node):        print('Extracted Double Linked list is:')        while head is not None:            print(head.data, end = ' ')            head = head.right # Driver program to test above functionif __name__ == '__main__':         """    Constructing below tree            5        // \\        3 6        // \\ \\        1 4 8    // \\ // \\    0 2 7 9    """    tree = BinaryTree()    tree.root = Node(5)    tree.root.left = Node(3)    tree.root.right = Node(6)    tree.root.left.left = Node(1)    tree.root.left.right = Node(4)    tree.root.right.right = Node(8)    tree.root.left.left.left = Node(0)    tree.root.left.left.right = Node(2)    tree.root.right.right.left = Node(7)    tree.root.right.right.right = Node(9)     tree.BToDll(tree.root)    tree.print_list(tree.head) # This code is contributed by Rajat Srivastava
// C# program to convert a given Binary Tree to Doubly Linked Listusing System; /* Structure for tree and Linked List */public class Node {    public int data;    public Node left, right;     public Node(int data)    {        this.data = data;        left = right = null;    }} class GFG {    // root    --> Root of Binary Tree    public Node root;     // head --> Pointer to head node of created doubly linked list    public Node head;     // A simple recursive function to convert a given    // Binary tree to Doubly Linked List    public virtual void BToDLL(Node root)    {        // Base cases        if (root == null)            return;         // Recursively convert right subtree        BToDLL(root.right);         // insert root into DLL        root.right = head;         // Change left pointer of previous head        if (head != null)            head.left = root;         // Change head of Doubly linked list        head = root;         // Recursively convert left subtree        BToDLL(root.left);    }     // Utility function for printing double linked list.    public virtual void printList(Node head)    {        Console.WriteLine("Extracted Double "                          + "Linked List is : ");        while (head != null) {            Console.Write(head.data + " ");            head = head.right;        }    }     // Driver program to test above function    public static void Main(string[] args)    {        /* Constructing below tree            5            / \            3     6        / \     \        1 4     8        / \     / \        0 2     7 9 */         GFG tree = new GFG();        tree.root = new Node(5);        tree.root.left = new Node(3);        tree.root.right = new Node(6);        tree.root.left.right = new Node(4);        tree.root.left.left = new Node(1);        tree.root.right.right = new Node(8);        tree.root.left.left.right = new Node(2);        tree.root.left.left.left = new Node(0);        tree.root.right.right.left = new Node(7);        tree.root.right.right.right = new Node(9);         tree.BToDLL(tree.root);        tree.printList(tree.head);    }} // This code is contributed by Shrikant13
<script>// javascript program to convert a given// Binary Tree to Doubly Linked List /* Structure for tree and Linked List */class Node {  constructor(data)    {        this.data = data;        this.left = this.right = null;    }}      // root    --> Root of Binary Tree    var root;     // head --> Pointer to head node of created doubly linked list    var head;     // A simple recursive function to convert a given    // Binary tree to Doubly Linked List    function BToDLL( root)    {        // Base cases        if (root == null)            return;         // Recursively convert right subtree        BToDLL(root.right);         // insert root into DLL        root.right = head;         // Change left pointer of previous head        if (head != null)            head.left = root;         // Change head of Doubly linked list        head = root;         // Recursively convert left subtree        BToDLL(root.left);    }     // Utility function for printing var linked list.    function printList( head)    {        document.write("Extracted Double Linked List is :<br/> ");        while (head != null) {            document.write(head.data + " ");            head = head.right;        }    }     // Driver program to test the above functions         /* Constructing below tree               5             /   \            3     6           / \     \          1   4     8         / \       / \        0   2     7   9  */         root = new Node(5);        root.left = new Node(3);        root.right = new Node(6);        root.left.right = new Node(4);        root.left.left = new Node(1);        root.right.right = new Node(8);        root.left.left.right = new Node(2);        root.left.left.left = new Node(0);        root.right.right.left = new Node(7);        root.right.right.right = new Node(9);         BToDLL(root);        printList(head); // This code contributed by umadevi9616</script>
Output : 
Extracted Double Linked list is:
0 1 2 3 4 5 6 7 8 9
Time Complexity: O(n), as the solution does a single traversal of given Binary Tree. 
YouTubeGeeksforGeeks501K subscribersConvert a given Binary Tree to Doubly Linked List | Set 4 | GeeksforGeeksWatch laterShareCopy linkInfoShoppingTap to unmuteIf playback doesn't begin shortly, try restarting your device.You're signed outVideos you watch may be added to the TV's watch history and influence TV recommendations. To avoid this, cancel and sign in to YouTube on your computer.CancelConfirmMore videosMore videosSwitch cameraShareInclude playlistAn error occurred while retrieving sharing information. Please try again later.Watch on0:000:000:00 / 5:05•Live•<div class="player-unavailable"><h1 class="message">An error occurred.</h1><div class="submessage"><a href="https://www.youtube.com/watch?v=lxeWqcTsmg0" target="_blank">Try watching this video on www.youtube.com</a>, or enable JavaScript if it is disabled in your browser.</div></div>
This article is contributed by Aditya Goel. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. 
shrikanth13
rajatsri94
mesajidiqbal
umadevi9616
Amazon
doubly linked list
Goldman Sachs
Microsoft
Linked List
Tree
Amazon
Microsoft
Goldman Sachs
Linked List
Tree
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
Comments
Old Comments
Linked List | Set 1 (Introduction)
Linked List | Set 2 (Inserting a node)
Stack Data Structure (Introduction and Program)
Linked List | Set 3 (Deleting a node)
LinkedList in Java
Tree Traversals (Inorder, Preorder and Postorder)
Binary Tree | Set 1 (Introduction)
Level Order Binary Tree Traversal
AVL Tree | Set 1 (Insertion)
Inorder Tree Traversal without Recursion