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# Convert Binary Tree to Doubly Linked List by keeping track of visited node

• Difficulty Level : Medium
• Last Updated : 16 Jan, 2023

Given a Binary Tree, The task is to convert it to a Doubly Linked List keeping the same order.

• 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. The following two different solutions have been discussed for this problem.
Convert a given Binary Tree to a Doubly Linked List | Set 1
Convert a given Binary Tree to a Doubly Linked List | Set 2

Approach: Below is the idea to solve the problem:

The idea is to do in-order traversal of the binary tree. While doing inorder traversal, keep track of the previously visited node in a variable, say prev. For every visited node, make it next to the prev and set previous of this node as prev. Below is the implementation of the above approach:

## C++

 `// A C++ program for in-place conversion of Binary Tree to` `// DLL` `#include ` `using` `namespace` `std;`   `/* A binary tree node has data, and left and right pointers` ` ``*/` `struct` `node {` `    ``int` `data;` `    ``node* left;` `    ``node* right;` `};`   `// 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` `void` `BinaryTree2DoubleLinkedList(node* root, node** head)` `{` `    ``// Base case` `    ``if` `(root == NULL)` `        ``return``;`   `    ``// Initialize previously visited node as NULL. This is` `    ``// static so that the same value is accessible in all` `    ``// recursive calls` `    ``static` `node* prev = NULL;`   `    ``// Recursively convert left subtree` `    ``BinaryTree2DoubleLinkedList(root->left, head);`   `    ``// Now convert this node` `    ``if` `(prev == NULL)` `        ``*head = root;` `    ``else` `{` `        ``root->left = prev;` `        ``prev->right = root;` `    ``}` `    ``prev = root;`   `    ``// Finally convert right subtree` `    ``BinaryTree2DoubleLinkedList(root->right, 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 = NULL;` `    ``BinaryTree2DoubleLinkedList(root, &head);`   `    ``// Print the converted list` `    ``printList(head);`   `    ``return` `0;` `}`   `// This code is contributed by Sania Kumari Gupta (kriSania804)`

## C

 `// A C program for in-place conversion of Binary Tree to DLL` `#include ` `#include `   `/* A binary tree node has data, and left and right pointers` ` ``*/` `typedef` `struct` `node {` `    ``int` `data;` `    ``struct` `node* left;` `    ``struct` `node* right;` `} 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` `// list` `void` `BinaryTree2DoubleLinkedList(node* root, node** head)` `{` `    ``// Base case` `    ``if` `(root == NULL)` `        ``return``;`   `    ``// Initialize previously visited node as NULL. This is` `    ``// static so that the same value is accessible in all` `    ``// recursive calls` `    ``static` `node* prev = NULL;`   `    ``// Recursively convert left subtree` `    ``BinaryTree2DoubleLinkedList(root->left, head);`   `    ``// Now convert this node` `    ``if` `(prev == NULL)` `        ``*head = root;` `    ``else` `{` `        ``root->left = prev;` `        ``prev->right = root;` `    ``}` `    ``prev = root;`   `    ``// Finally convert right subtree` `    ``BinaryTree2DoubleLinkedList(root->right, 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 = (node*)``malloc``(``sizeof``(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 = NULL;` `    ``BinaryTree2DoubleLinkedList(root, &head);`   `    ``// Print the converted list` `    ``printList(head);`   `    ``return` `0;` `}`   `// This code is contributed by Sania Kumari Gupta (kriSania804)`

## Java

 `// A Java program for in-place conversion of Binary Tree to DLL` ` `  `// A binary tree node has data, left pointers and right pointers` `class` `Node ` `{` `    ``int` `data;` `    ``Node left, right;` ` `  `    ``public` `Node(``int` `data) ` `    ``{` `        ``this``.data = data;` `        ``left = right = ``null``;` `    ``}` `}` ` `  `class` `BinaryTree ` `{` `    ``Node root;` `     `  `    ``// head --> Pointer to head node of created doubly linked list` `    ``Node head;` `     `  `    ``// Initialize previously visited node as NULL. This is` `    ``// static so that the same value is accessible in all recursive` `    ``// calls` `    ``static` `Node prev = ``null``;` ` `  `    ``// A simple recursive function to convert a given Binary tree ` `    ``// to Doubly Linked List` `    ``// root --> Root of Binary Tree` `    ``void` `BinaryTree2DoubleLinkedList(Node root) ` `    ``{` `        ``// Base case` `        ``if` `(root == ``null``)` `            ``return``;` ` `  `        ``// Recursively convert left subtree` `        ``BinaryTree2DoubleLinkedList(root.left);` ` `  `        ``// Now convert this node` `        ``if` `(prev == ``null``) ` `            ``head = root;` `        ``else` `        ``{` `            ``root.left = prev;` `            ``prev.right = root;` `        ``}` `        ``prev = root;` ` `  `        ``// Finally convert right subtree` `        ``BinaryTree2DoubleLinkedList(root.right);` `    ``}` ` `  `    ``/* 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) ` `    ``{` `        ``// Let us create the tree as shown in above diagram` `        ``BinaryTree tree = ``new` `BinaryTree();` `        ``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` `        ``tree.BinaryTree2DoubleLinkedList(tree.root);` `         `  `        ``// Print the converted List` `        ``tree.printList(tree.head);` ` `  `    ``}` `}` `// This code has been contributed by Mayank Jaiswal(mayank_24)`

## Python3

 `# Python program for conversion of` `# binary tree to doubly linked list.` `class` `Node:` `    ``def` `__init__(``self``, val):` `        ``self``.right ``=` `None` `        ``self``.data ``=` `val` `        ``self``.left ``=` `None`   `# Global variable used in convert` `prev ``=` `None`   `def` `BinaryTree2DoubleLinkedList(root):` `    `  `    ``# Base case` `    ``if` `root ``is` `None``:` `        ``return` `root` `            `  `    ``# Recursively convert left subtree` `    ``head ``=` `BinaryTree2DoubleLinkedList(root.left);` `    `  `    ``# Since we are going to change prev,` `    ``# we need to use global keyword` `    ``global` `prev` `    `  `    ``# If prev is empty, then this is the` `    ``# first node of DLL` `    ``if` `prev ``is` `None` `: ` `        ``head ``=` `root` `        `  `    ``else``:` `        ``root.left ``=` `prev` `        ``prev.right ``=` `root` `    `  `    ``# Update prev` `    ``prev ``=` `root; ` `    `  `    ``# Recursively convert right subtree` `    ``BinaryTree2DoubleLinkedList(root.right);` `    `  `    ``return` `head`   `def` `print_dll(head):` `    `  `    ``# Function to print nodes in given` `    ``# doubly linked list` `    ``while` `head ``is` `not` `None``:` `        ``print``(head.data, end``=``" "``)` `        ``head ``=` `head.right`     `# Driver program to test above functions` `# Let us create the tree as` `# shown in above diagram` `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 ``=` `BinaryTree2DoubleLinkedList(root)` `    `  `    ``# Print the converted list` `    ``print_dll(head)` `    `  `# This code is contributed by codesankalp (SANKALP)`

## C#

 `// A C# program for in-place conversion` `// of Binary Tree to DLL ` `using` `System;`   `// A binary tree node has data, left ` `// pointers and right pointers ` `public` `class` `Node` `{` `    ``public` `int` `data;` `    ``public` `Node left, right;`   `    ``public` `Node(``int` `data)` `    ``{` `        ``this``.data = data;` `        ``left = right = ``null``;` `    ``}` `}`   `class` `GFG` `{` `public` `Node root;`   `// head --> Pointer to head node of ` `// created doubly linked list ` `public` `Node head;`   `// Initialize previously visited node ` `// as NULL. This is static so that the` `// same value is accessible in all ` `// recursive calls ` `public` `static` `Node prev = ``null``;`   `// A simple recursive function to ` `// convert a given Binary tree ` `// to Doubly Linked List ` `// root --> Root of Binary Tree ` `public` `virtual` `void` `BinaryTree2DoubleLinkedList(Node root)` `{` `    ``// Base case ` `    ``if` `(root == ``null``)` `    ``{` `        ``return``;` `    ``}`   `    ``// Recursively convert left subtree ` `    ``BinaryTree2DoubleLinkedList(root.left);`   `    ``// Now convert this node ` `    ``if` `(prev == ``null``)` `    ``{` `        ``head = root;` `    ``}` `    ``else` `    ``{` `        ``root.left = prev;` `        ``prev.right = root;` `    ``}` `    ``prev = root;`   `    ``// Finally convert right subtree ` `    ``BinaryTree2DoubleLinkedList(root.right);` `}`   `/* 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 Code` `public` `static` `void` `Main(``string``[] args)` `{` `    ``// Let us create the tree as ` `    ``// shown in above diagram ` `    ``GFG tree = ``new` `GFG();` `    ``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 ` `    ``tree.BinaryTree2DoubleLinkedList(tree.root);`   `    ``// Print the converted List ` `    ``tree.printList(tree.head);`   `}` `}`   `// This code is contributed by Shrikant13`

## Javascript

 ``

Output

`25 12 30 10 36 15 `

Note: The use of static variables like above is not a recommended practice, here static is used for simplicity. Imagine if the same function is called for two or more trees. The old value of prev would be used in the next call for a different tree. To avoid such problems, we can use a double-pointer or a reference to a pointer.

Time Complexity: O(N), The above program does a simple inorder traversal, so time complexity is O(N) where N is the number of nodes in a given Binary tree.
Auxiliary Space: O(N), For recursion call stack.

## Convert a given Binary Tree to Doubly Linked List iteratively using Stack data structure:

Do iterative inorder traversal and maintain a prev pointer to point the last visited node then point current node’s perv to prev and prev’s next to current node.

Below is the implementation of the above approach:

## C++

 `// A C++ program for in-place conversion of Binary Tree to` `// DLL` `#include ` `using` `namespace` `std;`   `/* A binary tree node has data, and left and right pointers` ` ``*/` `struct` `node {` `    ``int` `data;` `    ``node* left;` `    ``node* right;` `};`   `node * bToDLL(node *root)` `{` `    ``stack> s;` `    ``s.push({root, 0});` `    ``vector<``int``> res;` `    ``bool` `flag = ``true``;` `    ``node* head = NULL;` `    ``node* prev = NULL;` `    ``while``(!s.empty()) {` `        ``auto` `x = s.top();` `        ``node* t = x.first;` `        ``int` `state = x.second;` `        ``s.pop();` `        ``if``(state == 3 or t == NULL) ``continue``;` `        ``s.push({t, state+1});` `        ``if``(state == 0) s.push({t->left, 0});` `        ``else` `if``(state == 1) {` `            ``if``(prev) prev->right = t;` `            ``t->left = prev;` `            ``prev = t;` `            ``if``(flag) {` `                ``head = t;` `                ``flag = ``false``;` `            ``}` `        ``}` `        ``else` `if``(state == 2) s.push({t->right, 0});` `    ``}` `    ``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 = bToDLL(root);`   `    ``// Print the converted list` `    ``printList(head);`   `    ``return` `0;` `}`

## Java

 `import` `java.util.Stack;`   `class` `Node {` `    ``int` `data;` `    ``Node left;` `    ``Node right;`   `  ``/* Helper function that allocates a new node with the` `   ``given data and NULL left and right pointers. */` `    ``public` `Node(``int` `data) {` `        ``this``.data = data;` `        ``left = ``null``;` `        ``right = ``null``;` `    ``}` `}`   `public` `class` `Main {` `    ``static` `Node bToDLL(Node root) {` `        ``Stack> s = ``new` `Stack<>();` `        ``s.push(``new` `Pair<>(root, ``0``));` `        ``boolean` `flag = ``true``;` `        ``Node head = ``null``;` `        ``Node prev = ``null``;` `        ``while` `(!s.empty()) {` `            ``Pair x = s.pop();` `            ``Node t = x.getKey();` `            ``int` `state = x.getValue();` `            ``if` `(state == ``3` `|| t == ``null``) {` `                ``continue``;` `            ``}` `            ``s.push(``new` `Pair<>(t, state + ``1``));` `            ``if` `(state == ``0``) {` `                ``s.push(``new` `Pair<>(t.left, ``0``));` `            ``} ``else` `if` `(state == ``1``) {` `                ``if` `(prev != ``null``) {` `                    ``prev.right = t;` `                ``}` `                ``t.left = prev;` `                ``prev = t;` `                ``if` `(flag) {` `                    ``head = t;` `                    ``flag = ``false``;` `                ``}` `            ``} ``else` `if` `(state == ``2``) {` `                ``s.push(``new` `Pair<>(t.right, ``0``));` `            ``}` `        ``}` `        ``return` `head;` `    ``}`   `  ``/* Function to print nodes in a given doubly linked list */` `    ``static` `void` `printList(Node head) {` `        ``while` `(head != ``null``) {` `            ``System.out.print(head.data + ``" "``);` `            ``head = head.right;` `        ``}` `    ``}`   `    ``public` `static` `void` `main(String[] args) ` `    ``{` `      `  `        ``// Let us create the tree shown in above diagram` `        ``Node 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` `        ``Node head = bToDLL(root);`   `        ``// Print the converted list` `        ``printList(head);` `    ``}` `}`   `class` `Pair {` `    ``private` `K key;` `    ``private` `V value;`   `    ``public` `Pair(K key, V value) {` `        ``this``.key = key;` `        ``this``.value = value;` `    ``}`   `    ``public` `K getKey() {` `        ``return` `key;` `    ``}`   `    ``public` `V getValue() {` `        ``return` `value;` `    ``}` `}`   `// This code is contributed by aadityamaharshi21.`

## Python3

 `def` `bToDLL(root):` `    ``s ``=` `[]` `    ``s.append([root, ``0``])` `    ``res ``=` `[]` `    ``flag ``=` `True` `    ``head ``=` `None` `    ``prev ``=` `None` `    ``while` `len``(s) > ``0``:` `        ``x ``=` `s.pop()` `        ``t ``=` `x[``0``]` `        ``state ``=` `x[``1``]` `        ``if` `state ``=``=` `3` `or` `t ``=``=` `None``: ``continue` `        ``s.append([t, state``+``1``])` `        ``if` `state ``=``=` `0``: s.append([t.left, ``0``])` `        ``elif` `state ``=``=` `1``: ` `            ``if` `prev !``=` `None``: prev.right ``=` `t` `            ``t.left ``=` `prev ` `            ``prev ``=` `t `   `            ``if` `flag:  ` `                ``head ``=` `t ` `                ``flag ``=` `False`    `        ``elif` `state ``=``=` `2``: s.append([t.right, ``0``])`   `    ``return` `head` `  `  `  ``# This code is contributed by Tapeshdua420.`

## C#

 `// C# code implementation for the above approach`   `using` `System;` `using` `System.Collections.Generic;`   `public` `class` `Node {` `  ``public` `int` `data;` `  ``public` `Node left;` `  ``public` `Node right;`   `  ``/* Helper function that allocates a new node with the` `    ``given data and NULL left and right pointers. */` `  ``public` `Node(``int` `data)` `  ``{` `    ``this``.data = data;` `    ``left = ``null``;` `    ``right = ``null``;` `  ``}` `}`   `public` `class` `GFG {`   `  ``static` `Node bToDLL(Node root)` `  ``{` `    ``Stack > s` `      ``= ``new` `Stack >();` `    ``s.Push(``new` `KeyValuePair(root, 0));` `    ``bool` `flag = ``true``;` `    ``Node head = ``null``;` `    ``Node prev = ``null``;` `    ``while` `(s.Count != 0) {` `      ``KeyValuePair x = s.Pop();` `      ``Node t = x.Key;` `      ``int` `state = x.Value;` `      ``if` `(state == 3 || t == ``null``) {` `        ``continue``;` `      ``}` `      ``s.Push(` `        ``new` `KeyValuePair(t, state + 1));` `      ``if` `(state == 0) {` `        ``s.Push(` `          ``new` `KeyValuePair(t.left, 0));` `      ``}` `      ``else` `if` `(state == 1) {` `        ``if` `(prev != ``null``) {` `          ``prev.right = t;` `        ``}` `        ``t.left = prev;` `        ``prev = t;` `        ``if` `(flag) {` `          ``head = t;` `          ``flag = ``false``;` `        ``}` `      ``}` `      ``else` `if` `(state == 2) {` `        ``s.Push(``new` `KeyValuePair(t.right,` `                                           ``0));` `      ``}` `    ``}` `    ``return` `head;` `  ``}`   `  ``/* Function to print nodes in a given doubly linked list` `     ``*/` `  ``static` `void` `printList(Node head)` `  ``{` `    ``while` `(head != ``null``) {` `      ``System.Console.Write(head.data + ``" "``);` `      ``head = head.right;` `    ``}` `  ``}`   `  ``static` `public` `void` `Main()` `  ``{`   `    ``// Code` `    ``// Let us create the tree shown in above diagram` `    ``Node 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` `    ``Node head = bToDLL(root);`   `    ``// Print the converted list` `    ``printList(head);` `  ``}` `}`   `// This code is contributed by lokesh.`

## Javascript

 `// JavaScript code for above approach`   `class Node {` `    ``constructor(data) {` `      ``this``.data = data;` `      ``this``.left = ``null``;` `      ``this``.right = ``null``;` `    ``}` `  ``}` `  `  `  ``function` `bToDLL(root) {` `    ``let s = [];` `    ``s.push([root, 0]);` `    ``let res = [];` `    ``let flag = ``true``;` `    ``let head = ``null``;` `    ``let prev = ``null``;` `  `  `    ``while` `(s.length > 0) {` `      ``let [t, state] = s.pop();` `      ``if` `(state === 3 || t === ``null``) ``continue``;` `      ``s.push([t, state + 1]);` `      ``if` `(state === 0) s.push([t.left, 0]);` `      ``else` `if` `(state === 1) {` `        ``if` `(prev) prev.right = t;` `        ``t.left = prev;` `        ``prev = t;` `        ``if` `(flag) {` `          ``head = t;` `          ``flag = ``false``;` `        ``}` `      ``} ``else` `if` `(state === 2) s.push([t.right, 0]);` `    ``}` `  `  `    ``return` `head;` `  ``}` `  `  `  ``function` `newNode(data) {` `    ``let new_node = ``new` `Node(data);` `    ``return` `new_node;` `  ``}` `  `  `  ``function` `printList(node) {` `    ``while` `(node !== ``null``) {` `      ``console.log(node.data);` `      ``node = node.right;` `    ``}` `  ``}` `  `  `  ``let 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);` `  `  `  ``let head = bToDLL(root);` `  ``printList(head);`   `// This code is contributed by adityamaharshi21`

Output

`25 12 30 10 36 15 `

Time complexity: O(N)
Auxiliary Space: O(N)