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Convert a given Binary Tree to Doubly Linked List | Set 1

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  • Difficulty Level : Hard
  • Last Updated : 07 Jul, 2022

Given a Binary Tree (Bt), convert it to a Doubly Linked List(DLL). 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 the same as in Inorder for the given Binary Tree. The first node of Inorder traversal (leftmost node in BT) must be the head node of the DLL.

TreeToList

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The problem here is simpler as we don’t need to create a circular DLL, but a simple DLL. The idea behind its solution is quite simple and straight.

  1. If the left subtree exists, process the left subtree
    1. Recursively convert the left subtree to DLL.
    2. Then find the inorder predecessor of the root in the left subtree (the inorder predecessor is the rightmost node in the left subtree).
    3. Make the inorder predecessor as the previous root and the root as the next in order predecessor.
  2.  If the right subtree exists, process the right subtree (Below 3 steps are similar to the left subtree).
    1. Recursively convert the right subtree to DLL.
    2. Then find the inorder successor of the root in the right subtree (in order the successor is the leftmost node in the right subtree).
    3. Make the inorder successor as the next root and the root as the previous inorder successor.
  3. Find the leftmost node and return it (the leftmost node is always the head of a converted DLL).

Below is the source code for the above algorithm.

C++




// A C++ program for in-place
// conversion of Binary Tree to DLL
#include <bits/stdc++.h>
using namespace std;
  
/* A binary tree node has data,
and left and right pointers */
class node {
public:
    int data;
    node* left;
    node* right;
};
  
/* This is the core function to convert
Tree to list. This function follows
steps 1 and 2 of the above algorithm */
node* bintree2listUtil(node* root)
{
    // Base case
    if (root == NULL)
        return root;
  
    // Convert the left subtree and link to root
    if (root->left != NULL) {
        // Convert the left subtree
        node* left = bintree2listUtil(root->left);
  
        // Find inorder predecessor. After this loop, left
        // will point to the inorder predecessor
        for (; left->right != NULL; left = left->right)
            ;
  
        // Make root as next of the predecessor
        left->right = root;
  
        // Make predecessor as previous of root
        root->left = left;
    }
  
    // Convert the right subtree and link to root
    if (root->right != NULL) {
        // Convert the right subtree
        node* right = bintree2listUtil(root->right);
  
        // Find inorder successor. After this loop, right
        // will point to the inorder successor
        for (; right->left != NULL; right = right->left)
            ;
  
        // Make root as previous of successor
        right->left = root;
  
        // Make successor as next of root
        root->right = right;
    }
  
    return root;
}
  
// The main function that first calls
// bintree2listUtil(), then follows step 3
// of the above algorithm
node* bintree2list(node* root)
{
    // Base case
    if (root == NULL)
        return root;
  
    // Convert to DLL using bintree2listUtil()
    root = bintree2listUtil(root);
  
    // bintree2listUtil() returns root node of the converted
    // DLL. We need pointer to the leftmost node which is
    // head of the constructed DLL, so move to the leftmost
    // node
    while (root->left != NULL)
        root = root->left;
  
    return (root);
}
  
/* Helper function that allocates a new node with the
given data and NULL left and right pointers. */
node* newNode(int data)
{
    node* new_node = new node();
    new_node->data = data;
    new_node->left = new_node->right = NULL;
    return (new_node);
}
  
/* Function to print nodes in a given doubly linked list */
void printList(node* node)
{
    while (node != NULL) {
        cout << node->data << " ";
        node = node->right;
    }
}
  
/* Driver code*/
int main()
{
    // Let us create the tree shown in above diagram
    node* root = newNode(10);
    root->left = newNode(12);
    root->right = newNode(15);
    root->left->left = newNode(25);
    root->left->right = newNode(30);
    root->right->left = newNode(36);
  
    // Convert to DLL
    node* head = bintree2list(root);
  
    // Print the converted list
    printList(head);
  
    return 0;
}
  
// This code is contributed by rathbhupendra


C




// A C program for in-place conversion of Binary Tree to DLL
#include <stdio.h>
  
/* A binary tree node has data, and left and right pointers */
struct node
{
    int data;
    node* left;
    node* right;
};
  
/* This is the core function to convert Tree to list. This function follows
  steps 1 and 2 of the above algorithm */
node* bintree2listUtil(node* root)
{
    // Base case
    if (root == NULL)
        return root;
  
    // Convert the left subtree and link to root
    if (root->left != NULL)
    {
        // Convert the left subtree
        node* left = bintree2listUtil(root->left);
  
        // Find inorder predecessor. After this loop, left
        // will point to the inorder predecessor
        for (; left->right!=NULL; left=left->right);
  
        // Make root as next of the predecessor
        left->right = root;
  
        // Make predecessor as previous of root
        root->left = left;
    }
  
    // Convert the right subtree and link to root
    if (root->right!=NULL)
    {
        // Convert the right subtree
        node* right = bintree2listUtil(root->right);
  
        // Find inorder successor. After this loop, right
        // will point to the inorder successor
        for (; right->left!=NULL; right = right->left);
  
        // Make root as previous of successor
        right->left = root;
  
        // Make successor as next of root
        root->right = right;
    }
  
    return root;
}
  
// The main function that first calls bintree2listUtil(), then follows step 3 
//  of the above algorithm
node* bintree2list(node *root)
{
    // Base case
    if (root == NULL)
        return root;
  
    // Convert to DLL using bintree2listUtil()
    root = bintree2listUtil(root);
  
    // bintree2listUtil() returns root node of the converted
    // DLL.  We need pointer to the leftmost node which is
    // head of the constructed DLL, so move to the leftmost node
    while (root->left != NULL)
        root = root->left;
  
    return (root);
}
  
/* Helper function that allocates a new node with the
   given data and NULL left and right pointers. */
node* newNode(int data)
{
    node* new_node = new node;
    new_node->data = data;
    new_node->left = new_node->right = NULL;
    return (new_node);
}
  
/* Function to print nodes in a given doubly linked list */
void printList(node *node)
{
    while (node!=NULL)
    {
        printf("%d ", node->data);
        node = node->right;
    }
}
  
/* Driver program to test above functions*/
int main()
{
    // Let us create the tree shown in above diagram
    node *root        = newNode(10);
    root->left        = newNode(12);
    root->right       = newNode(15);
    root->left->left  = newNode(25);
    root->left->right = newNode(30);
    root->right->left = newNode(36);
  
    // Convert to DLL
    node *head = bintree2list(root);
  
    // Print the converted list
    printList(head);
  
    return 0;
}


Java




// Java program to convert binary tree to double linked list
   
/* A binary tree node has data, and left and right pointers */
class Node 
{
    int data;
    Node left, right;
   
    Node(int item) 
    {
        data = item;
        left = right = null;
    }
}
   
class BinaryTree 
{
    Node root;
    /* This is the core function to convert Tree to list. This function
       follows steps 1 and 2 of the above algorithm */
   
    Node bintree2listUtil(Node node) 
    {
        // Base case
        if (node == null)
            return node;
   
        // Convert the left subtree and link to root
        if (node.left != null
        {
            // Convert the left subtree
            Node left = bintree2listUtil(node.left);
   
            // Find inorder predecessor. After this loop, left
            // will point to the inorder predecessor
            for (; left.right != null; left = left.right);
   
            // Make root as next of the predecessor
            left.right = node;
   
            // Make predecessor as previous of root
            node.left = left;
        }
   
        // Convert the right subtree and link to root
        if (node.right != null
        {
            // Convert the right subtree
            Node right = bintree2listUtil(node.right);
   
            // Find inorder successor. After this loop, right
            // will point to the inorder successor
            for (; right.left != null; right = right.left);
   
            // Make root as previous of successor
            right.left = node;
   
            // Make successor as next of root
            node.right = right;
        }
   
        return node;
    }
   
    // The main function that first calls bintree2listUtil(), then follows
    // step 3 of the above algorithm
       
    Node bintree2list(Node node) 
    {
        // Base case
        if (node == null)
            return node;
   
        // Convert to DLL using bintree2listUtil()
        node = bintree2listUtil(node);
   
        // bintree2listUtil() returns root node of the converted
        // DLL.  We need pointer to the leftmost node which is
        // head of the constructed DLL, so move to the leftmost node
        while (node.left != null)
            node = node.left;
   
        return node;
    }
   
    /* Function to print nodes in a given doubly linked list */
    void printList(Node node) 
    {
        while (node != null
        {
            System.out.print(node.data + " ");
            node = node.right;
        }
    }
   
    /* Driver program to test above functions*/
    public static void main(String[] args) 
    {
        BinaryTree tree = new BinaryTree();
   
        // Let us create the tree shown in above diagram
        tree.root = new Node(10);
        tree.root.left = new Node(12);
        tree.root.right = new Node(15);
        tree.root.left.left = new Node(25);
        tree.root.left.right = new Node(30);
        tree.root.right.left = new Node(36);
   
        // Convert to DLL
        Node head = tree.bintree2list(tree.root);
   
        // Print the converted list
        tree.printList(head);
    }
}


Python3




# Python program to convert 
# binary tree to doubly linked list
  
class Node(object):
      
    """Binary tree Node class has 
    data, left and right child"""
    def __init__(self, item):
        self.data = item
        self.left = None
        self.right = None
  
def BTToDLLUtil(root):
      
    """This is a utility function to 
    convert the binary tree to doubly 
    linked list. Most of the core task
    is done by this function."""
    if root is None:
        return root
  
    # Convert left subtree 
    # and link to root
    if root.left:
          
        # Convert the left subtree
        left = BTToDLLUtil(root.left)
  
        # Find inorder predecessor, After 
        # this loop, left will point to the 
        # inorder predecessor of root
        while left.right:
            left = left.right
  
        # Make root as next of predecessor
        left.right = root
          
        # Make predecessor as 
        # previous of root
        root.left = left
  
    # Convert the right subtree
    # and link to root
    if root.right:
          
        # Convert the right subtree
        right = BTToDLLUtil(root.right)
  
        # Find inorder successor, After 
        # this loop, right will point to 
        # the inorder successor of root
        while right.left:
            right = right.left
  
        # Make root as previous 
        # of successor
        right.left = root
          
        # Make successor as 
        # next of root
        root.right = right
  
    return root
  
def BTToDLL(root):
    if root is None:
        return root
  
    # Convert to doubly linked 
    # list using BLLToDLLUtil
    root = BTToDLLUtil(root)
      
    # We need pointer to left most 
    # node which is head of the 
    # constructed Doubly Linked list
    while root.left:
        root = root.left
  
    return root
  
def print_list(head):
      
    """Function to print the given
       doubly linked list"""
    if head is None:
        return
    while head:
        print(head.data, end = " ")
        head = head.right
  
# Driver Code
if __name__ == '__main__':
    root = Node(10)
    root.left = Node(12)
    root.right = Node(15)
    root.left.left = Node(25)
    root.left.right = Node(30)
    root.right.left = Node(36)
  
    head = BTToDLL(root)
    print_list(head)
  
# This code is contributed 
# by viveksyngh


C#




using System;
  
// C# program to convert binary tree to double linked list 
  
/* A binary tree node has data, and left and right pointers */
public class Node
{
    public int data;
    public Node left, right;
  
    public Node(int item)
    {
        data = item;
        left = right = null;
    }
}
  
public class BinaryTree
{
    public Node root;
    /* This is the core function to convert Tree to list. This function 
       follows steps 1 and 2 of the above algorithm */
  
    public virtual Node bintree2listUtil(Node node)
    {
        // Base case 
        if (node == null)
        {
            return node;
        }
  
        // Convert the left subtree and link to root 
        if (node.left != null)
        {
            // Convert the left subtree 
            Node left = bintree2listUtil(node.left);
  
            // Find inorder predecessor. After this loop, left 
            // will point to the inorder predecessor 
            for (; left.right != null; left = left.right)
            {
                ;
            }
  
            // Make root as next of the predecessor 
            left.right = node;
  
            // Make predecessor as previous of root 
            node.left = left;
        }
  
        // Convert the right subtree and link to root 
        if (node.right != null)
        {
            // Convert the right subtree 
            Node right = bintree2listUtil(node.right);
  
            // Find inorder successor. After this loop, right 
            // will point to the inorder successor 
            for (; right.left != null; right = right.left)
            {
                ;
            }
  
            // Make root as previous of successor 
            right.left = node;
  
            // Make successor as next of root 
            node.right = right;
        }
  
        return node;
    }
  
    // The main function that first calls bintree2listUtil(), then follows 
    // step 3 of the above algorithm 
  
    public virtual Node bintree2list(Node node)
    {
        // Base case 
        if (node == null)
        {
            return node;
        }
  
        // Convert to DLL using bintree2listUtil() 
        node = bintree2listUtil(node);
  
        // bintree2listUtil() returns root node of the converted 
        // DLL.  We need pointer to the leftmost node which is 
        // head of the constructed DLL, so move to the leftmost node 
        while (node.left != null)
        {
            node = node.left;
        }
  
        return node;
    }
  
    /* Function to print nodes in a given doubly linked list */
    public virtual void printList(Node node)
    {
        while (node != null)
        {
            Console.Write(node.data + " ");
            node = node.right;
        }
    }
  
    /* Driver program to test above functions*/
    public static void Main(string[] args)
    {
        BinaryTree tree = new BinaryTree();
  
        // Let us create the tree shown in above diagram 
        tree.root = new Node(10);
        tree.root.left = new Node(12);
        tree.root.right = new Node(15);
        tree.root.left.left = new Node(25);
        tree.root.left.right = new Node(30);
        tree.root.right.left = new Node(36);
  
        // Convert to DLL 
        Node head = tree.bintree2list(tree.root);
  
        // Print the converted list 
        tree.printList(head);
    }
}
  
// This code is contributed by Shrikant13


Javascript




<script>
    // javascript program to convert binary tree to double linked list
  
    /* A binary tree node has data, and left and right pointers */
    class Node {
        constructor(val) {
            this.data = val;
            this.left = null;
            this.right = null;
        }
    }
    var root;
    /*
     * This is the core function to convert Tree to list. This function follows
     * steps 1 and 2 of the above algorithm
     */
  
    function bintree2listUtil(node) {
        // Base case
        if (node == null)
            return node;
  
        // Convert the left subtree and link to root
        if (node.left != null) {
            // Convert the left subtree
            var left = bintree2listUtil(node.left);
  
            // Find inorder predecessor. After this loop, left
            // will point to the inorder predecessor
            for (; left.right != null; left = left.right)
                   
  
            // Make root as next of the predecessor
            left.right = node;
  
            // Make predecessor as previous of root
            node.left = left;
        }
  
        // Convert the right subtree and link to root
        if (node.right != null) {
            // Convert the right subtree
            var right = bintree2listUtil(node.right);
  
            // Find inorder successor. After this loop, right
            // will point to the inorder successor
            for (; right.left != null; right = right.left)
                  
  
            // Make root as previous of successor
            right.left = node;
  
            // Make successor as next of root
            node.right = right;
        }
  
        return node;
    }
  
    // The main function that first calls bintree2listUtil(), then follows
    // step 3 of the above algorithm
  
    function bintree2list(node) {
        // Base case
        if (node == null)
            return node;
  
        // Convert to DLL using bintree2listUtil()
        node = bintree2listUtil(node);
  
        // bintree2listUtil() returns root node of the converted
        // DLL. We need pointer to the leftmost node which is
        // head of the constructed DLL, so move to the leftmost node
        while (node.left != null)
            node = node.left;
  
        return node;
    }
  
    /* Function to print nodes in a given doubly linked list */
    function printList(node) {
        while (node != null) {
            document.write(node.data + " ");
            node = node.right;
        }
    }
  
    /* Driver program to test above functions */
      
    // Let us create the tree shown in above diagram
    root = new Node(10);
    root.left = new Node(12);
    root.right = new Node(15);
    root.left.left = new Node(25);
    root.left.right = new Node(30);
    root.right.left = new Node(36);
  
    // Convert to DLL
    var head = bintree2list(root);
  
    // Print the converted list
    printList(head);
  
// This code contributed by umadevi9616 
</script>


Output

25 12 30 10 36 15 

Another Approach:
Algorithm:

  1. Traverse the tree in inorder fashion.
  2. While visiting each node, keep track of DLL’s head and tail pointers, insert each visited node to the end of DLL using tail pointer.
  3. Return head of the list.

Below is the implementation of the above approach:

C++




// A C++ program for in-place
// conversion of Binary Tree to DLL
#include <bits/stdc++.h>
using namespace std;
  
/* A binary tree node has data,
and left and right pointers */
class node {
public:
    int data;
    node* left;
    node* right;
};
  
/* This is the core function to convert
Tree to list.*/
void bintree2listUtil(node* root, node** head, node** tail)
{
    if (root == NULL)
        return;
  
    bintree2listUtil(root->left, head, tail);
  
    if (*head == NULL) {
        *head = root;
    }
  
    root->left = *tail;
  
    if (*tail != NULL) {
        (*tail)->right = root;
    }
  
    *tail = root;
  
    bintree2listUtil(root->right, head, tail);
}
  
// The main function that first calls
// bintree2listUtil()
node* bintree2list(node* root)
{
    // Base case
    if (root == NULL)
        return root;
  
    node* head = NULL;
    node* tail = NULL;
  
    bintree2listUtil(root, &head, &tail);
  
    return head;
}
  
/* Helper function that allocates a new node with the
given data and NULL left and right pointers. */
node* newNode(int data)
{
    node* new_node = new node();
    new_node->data = data;
    new_node->left = new_node->right = NULL;
    return (new_node);
}
  
/* Function to print nodes in a given doubly linked list */
void printList(node* node)
{
    while (node != NULL) {
        cout << node->data << " ";
        node = node->right;
    }
}
  
/* Driver code*/
int main()
{
    // Let us create the tree shown in above diagram
    node* root = newNode(10);
    root->left = newNode(12);
    root->right = newNode(15);
    root->left->left = newNode(25);
    root->left->right = newNode(30);
    root->right->left = newNode(36);
  
    // Convert to DLL
    node* head = bintree2list(root);
  
    // Print the converted list
    printList(head);
  
    return 0;
}
  
// This code is contributed by rathbhupendra


Java




// A Java program for in-place
// conversion of Binary Tree to DLL
import java.util.*;
  
class GFG{
  
/* A binary tree node has data,
and left and right pointers */
static class node {
  
    int data;
    node left;
    node right;
};
  
    /*
     * This is the core function to convert Tree to list.
     */
static node head, tail;
static void bintree2listUtil(node root)
{
    if (root == null)
        return ;
  
    node left = root.left;
    node right = root.right;
  
    bintree2listUtil(root.left);
  
    if (head == null) {
        head = root;
    }
  
    root.left = tail;
  
    if (tail != null) {
        (tail).right = root;
    }
  
    tail = root;
  
    bintree2listUtil(root.right);
}
  
// The main function that first calls
// bintree2listUtil()
static void bintree2list(node root)
{
    
    // Base case
    if (root == null)
        head= root;
  
    
  
    bintree2listUtil(root);
  
    }
  
/* Helper function that allocates a new node with the
given data and null left and right pointers. */
static node newNode(int data)
{
    node new_node = new node();
    new_node.data = data;
    new_node.left = new_node.right = null;
    return (new_node);
}
  
    /* Function to print nodes in a given doubly linked list */
static void printList()
{
    while (head != null) {
        System.out.print(head.data+ " ");
        head = head.right;
    }
}
  
    /* Driver code */
public static void main(String[] args)
{
    
    // Let us create the tree shown in above diagram
    node root = newNode(10);
    root.left = newNode(12);
    root.right = newNode(15);
    root.left.left = newNode(25);
    root.left.right = newNode(30);
    root.right.left = newNode(36);
    head = null;
    tail = null;
    
    // Convert to DLL
    bintree2list(root);
  
    // Print the converted list
    printList();
  
}
}
  
// This code is contributed by umadevi9616 


C#




// A C# program for in-place
// conversion of Binary Tree to DLL
using System;
  
public class GFG {
  
    /*
     * A binary tree node has data, and left and right pointers
     */
public    class node {
  
    public    int data;
    public    node left;
    public    node right;
    };
  
    /*
     * This is the core function to convert Tree to list.
     */
    static node head, tail;
  
    static void bintree2listUtil(node root) {
        if (root == null)
            return;
  
        node left = root.left;
        node right = root.right;
  
        bintree2listUtil(root.left);
  
        if (head == null) {
            head = root;
        }
  
        root.left = tail;
  
        if (tail != null) {
            (tail).right = root;
        }
  
        tail = root;
  
        bintree2listUtil(root.right);
    }
  
    // The main function that first calls
    // bintree2listUtil()
    static void bintree2list(node root) {
  
        // Base case
        if (root == null)
            head = root;
  
        bintree2listUtil(root);
  
    }
  
    /*
     * Helper function that allocates a new node with the given data and null left
     * and right pointers.
     */
    static node newNode(int data) {
        node new_node = new node();
        new_node.data = data;
        new_node.left = new_node.right = null;
        return (new_node);
    }
  
    /* Function to print nodes in a given doubly linked list */
    static void printList() {
        while (head != null) {
            Console.Write(head.data + " ");
            head = head.right;
        }
    }
  
    /* Driver code */
    public static void Main(String[] args) {
  
        // Let us create the tree shown in above diagram
        node root = newNode(10);
        root.left = newNode(12);
        root.right = newNode(15);
        root.left.left = newNode(25);
        root.left.right = newNode(30);
        root.right.left = newNode(36);
        head = null;
        tail = null;
  
        // Convert to DLL
        bintree2list(root);
  
        // Print the converted list
        printList();
  
    }
}
  
// This code is contributed by gauravrajput1


Javascript




<script>
// A javascript program for in-place
// conversion of Binary Tree to DLL
  
    /*
     * A binary tree node has data, and left and right pointers
     */
     class Node {
        constructor() {
            this.data = 0;
            this.left = null;
            this.right = null;
        }
    }
    /*
     * This is the core function to convert Tree to list.
     */
     var head, tail;
  
    function bintree2listUtil(root) {
        if (root == null)
            return;
  
        var left = root.left;
        var right = root.right;
  
        bintree2listUtil(root.left);
  
        if (head == null) {
            head = root;
        }
  
        root.left = tail;
  
        if (tail != null) {
            (tail).right = root;
        }
  
        tail = root;
  
        bintree2listUtil(root.right);
    }
  
    // The main function that first calls
    // bintree2listUtil()
    function bintree2list( root) {
  
        // Base case
        if (root == null)
            head = root;
  
        bintree2listUtil(root);
  
    }
  
    /*
     * Helper function that allocates a new node with the given data and null left
     * and right pointers.
     */
     function newNode(data) {
        var new_node = new Node();
        new_node.data = data;
        new_node.left = new_node.right = null;
        return (new_node);
    }
  
    /* Function to print nodes in a given doubly linked list */
    function printList() {
        while (head != null) {
            document.write(head.data + " ");
            head = head.right;
        }
    }
  
    /* Driver code */
      
        // Let us create the tree shown in above diagram
        var root = newNode(10);
        root.left = newNode(12);
        root.right = newNode(15);
        root.left.left = newNode(25);
        root.left.right = newNode(30);
        root.right.left = newNode(36);
        head = null;
        tail = null;
  
        // Convert to DLL
        bintree2list(root);
  
        // Print the converted list
        printList();
  
// This code is contributed by gauravrajput1 
</script>


Output

25 12 30 10 36 15 

This article is compiled by Ashish Mangla and reviewed by GeeksforGeeks team. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
You may also like to see Convert a given Binary Tree to Doubly Linked List | Set 2 for another simple and efficient solution.
 


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