# Convert a Binary Tree to a Circular Doubly Link List

• Difficulty Level : Hard
• Last Updated : 21 Jun, 2022

Given a Binary Tree, convert it to a Circular Doubly Linked List (In-Place).

• The left and right pointers in nodes are to be used as previous and next pointers respectively in converted Circular Linked List.
• The order of nodes in the List must be the same as in Inorder for the given Binary Tree.
• The first node of Inorder traversal must be the head node of the Circular List.

Example: The idea can be described using the below steps.
1) Write a general-purpose function that concatenates two given circular doubly lists (This function is explained below).
2) Now traverse the given tree
….a) Recursively convert left subtree to a circular DLL. Let the converted list be leftList.
….a) Recursively convert right subtree to a circular DLL. Let the converted list be rightList.
….c) Make a circular linked list of root of the tree, make left and right of root to point to itself.
….d) Concatenate leftList with the list of the single root node.
….e) Concatenate the list produced in the step above (d) with rightList.
Note that the above code traverses the tree in Postorder fashion. We can traverse in an inorder fashion also. We can first concatenate left subtree and root, then recur for the right subtree and concatenate the result with left-root concatenation.

How to Concatenate two circular DLLs?

• Get the last node of the left list. Retrieving the last node is an O(1) operation since the prev pointer of the head points to the last node of the list.
• Connect it with the first node of the right list
• Get the last node of the second list
• Connect it with the head of the list.

Below are implementations of the above idea.

## C++

 `// C++ Program to convert a Binary Tree` `// to a Circular Doubly Linked List` `#include` `using` `namespace` `std;`   `// To represents a node of a Binary Tree` `struct` `Node` `{` `    ``struct` `Node *left, *right;` `    ``int` `data;` `};`   `// A function that appends rightList at the end` `// of leftList.` `Node *concatenate(Node *leftList, Node *rightList)` `{` `    ``// If either of the list is empty` `    ``// then return the other list` `    ``if` `(leftList == NULL)` `        ``return` `rightList;` `    ``if` `(rightList == NULL)` `        ``return` `leftList;`   `    ``// Store the last Node of left List` `    ``Node *leftLast = leftList->left;`   `    ``// Store the last Node of right List` `    ``Node *rightLast = rightList->left;`   `    ``// Connect the last node of Left List` `    ``// with the first Node of the right List` `    ``leftLast->right = rightList;` `    ``rightList->left = leftLast;`   `    ``// Left of first node points to` `    ``// the last node in the list` `    ``leftList->left = rightLast;`   `    ``// Right of last node refers to the first` `    ``// node of the List` `    ``rightLast->right = leftList;`   `    ``return` `leftList;` `}`   `// Function converts a tree to a circular Linked List` `// and then returns the head of the Linked List` `Node *bTreeToCList(Node *root)` `{` `    ``if` `(root == NULL)` `        ``return` `NULL;`   `    ``// Recursively convert left and right subtrees` `    ``Node *left = bTreeToCList(root->left);` `    ``Node *right = bTreeToCList(root->right);`   `    ``// Make a circular linked list of single node` `    ``// (or root). To do so, make the right and` `    ``// left pointers of this node point to itself` `    ``root->left = root->right = root;`   `    ``// Step 1 (concatenate the left list with the list ` `    ``//         with single node, i.e., current node)` `    ``// Step 2 (concatenate the returned list with the` `    ``//         right List)` `    ``return` `concatenate(concatenate(left, root), right);` `}`   `// Display Circular Link List` `void` `displayCList(Node *head)` `{` `    ``cout << ``"Circular Linked List is :\n"``;` `    ``Node *itr = head;` `    ``do` `    ``{` `        ``cout << itr->data <<``" "``;` `        ``itr = itr->right;` `    ``} ``while` `(head!=itr);` `    ``cout << ``"\n"``;` `}`     `// Create a new Node and return its address` `Node *newNode(``int` `data)` `{` `    ``Node *temp = ``new` `Node();` `    ``temp->data = data;` `    ``temp->left = temp->right = NULL;` `    ``return` `temp;` `}`   `// Driver Program to test above function` `int` `main()` `{` `    ``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);`   `    ``Node *head = bTreeToCList(root);` `    ``displayCList(head);`   `    ``return` `0;` `}`

## Java

 `// Java Program to convert a Binary Tree to a` `// Circular Doubly Linked List`   `// Node class represents a Node of a Tree` `class` `Node` `{` `    ``int` `val;` `    ``Node left,right;`   `    ``public` `Node(``int` `val)` `    ``{` `        ``this``.val = val;` `        ``left = right = ``null``;` `    ``}` `}`   `// A class to represent a tree` `class` `Tree` `{` `    ``Node root;` `    ``public` `Tree()` `    ``{` `        ``root = ``null``;` `    ``}`   `    ``// concatenate both the lists and returns the head` `    ``// of the List` `    ``public` `Node concatenate(Node leftList,Node rightList)` `    ``{` `        ``// If either of the list is empty, then` `        ``// return the other list` `        ``if` `(leftList == ``null``)` `            ``return` `rightList;` `        ``if` `(rightList == ``null``)` `            ``return` `leftList;`   `        ``// Store the last Node of left List` `        ``Node leftLast = leftList.left;`   `        ``// Store the last Node of right List` `        ``Node rightLast = rightList.left;`   `        ``// Connect the last node of Left List` `        ``// with the first Node of the right List` `        ``leftLast.right = rightList;` `        ``rightList.left = leftLast;`   `        ``// left of first node refers to` `        ``// the last node in the list` `        ``leftList.left = rightLast;`   `        ``// Right of last node refers to the first` `        ``// node of the List` `        ``rightLast.right = leftList;`   `        ``// Return the Head of the List` `        ``return` `leftList;` `    ``}`   `    ``// Method converts a tree to a circular` `    ``// Link List and then returns the head` `    ``// of the Link List` `    ``public` `Node bTreeToCList(Node root)` `    ``{` `        ``if` `(root == ``null``)` `            ``return` `null``;`   `        ``// Recursively convert left and right subtrees` `        ``Node left = bTreeToCList(root.left);` `        ``Node right = bTreeToCList(root.right);`   `        ``// Make a circular linked list of single node` `        ``// (or root). To do so, make the right and` `        ``// left pointers of this node point to itself` `        ``root.left = root.right = root;`   `        ``// Step 1 (concatenate the left list with the list ` `        ``//         with single node, i.e., current node)` `        ``// Step 2 (concatenate the returned list with the` `        ``//         right List)` `        ``return` `concatenate(concatenate(left, root), right);` `    ``}`   `    ``// Display Circular Link List` `    ``public` `void` `display(Node head)` `    ``{` `        ``System.out.println(``"Circular Linked List is :"``);` `        ``Node itr = head;` `        ``do` `        ``{` `            ``System.out.print(itr.val+ ``" "` `);` `            ``itr = itr.right;` `        ``}` `        ``while` `(itr != head);` `        ``System.out.println();` `    ``}` `}`   `// Driver Code` `class` `Main` `{` `    ``public` `static` `void` `main(String args[])` `    ``{` `        ``// Build the tree` `        ``Tree tree = ``new` `Tree();` `        ``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``);`   `        ``// head refers to the head of the Link List` `        ``Node head = tree.bTreeToCList(tree.root);`   `        ``// Display the Circular LinkedList` `        ``tree.display(head);` `    ``}` `}`

## Python3

 `# Python3 Program to convert a Binary ` `# Tree to a Circular Doubly Linked List `   `class` `newNode:` `    ``def` `__init__(``self``, data):` `        ``self``.data ``=` `data` `        ``self``.left ``=` `self``.right ``=` `None` `        `  `# A function that appends rightList ` `# at the end of leftList. ` `def` `concatenate(leftList, rightList):` `    `  `    ``# If either of the list is empty ` `    ``# then return the other list ` `    ``if` `(leftList ``=``=` `None``):` `        ``return` `rightList ` `    ``if` `(rightList ``=``=` `None``): ` `        ``return` `leftList `   `    ``# Store the last Node of left List ` `    ``leftLast ``=` `leftList.left `   `    ``# Store the last Node of right List ` `    ``rightLast ``=` `rightList.left `   `    ``# Connect the last node of Left List ` `    ``# with the first Node of the right List ` `    ``leftLast.right ``=` `rightList ` `    ``rightList.left ``=` `leftLast `   `    ``# Left of first node points to ` `    ``# the last node in the list ` `    ``leftList.left ``=` `rightLast `   `    ``# Right of last node refers to ` `    ``# the first node of the List ` `    ``rightLast.right ``=` `leftList `   `    ``return` `leftList`   `# Function converts a tree to a circular ` `# Linked List and then returns the head ` `# of the Linked List ` `def` `bTreeToCList(root):` `    ``if` `(root ``=``=` `None``): ` `        ``return` `None`   `    ``# Recursively convert left and ` `    ``# right subtrees ` `    ``left ``=` `bTreeToCList(root.left) ` `    ``right ``=` `bTreeToCList(root.right) `   `    ``# Make a circular linked list of single ` `    ``# node (or root). To do so, make the ` `    ``# right and left pointers of this node` `    ``# point to itself ` `    ``root.left ``=` `root.right ``=` `root `   `    ``# Step 1 (concatenate the left list ` `    ``#          with the list with single ` `    ``#         node, i.e., current node) ` `    ``# Step 2 (concatenate the returned list` `    ``#          with the right List) ` `    ``return` `concatenate(concatenate(left, ` `                            ``root), right)`   `# Display Circular Link List ` `def` `displayCList(head):` `    ``print``(``"Circular Linked List is :"``) ` `    ``itr ``=` `head` `    ``first ``=` `1` `    ``while` `(head !``=` `itr ``or` `first):` `        ``print``(itr.data, end ``=` `" "``) ` `        ``itr ``=` `itr.right` `        ``first ``=` `0` `    ``print``()`   `# Driver Code` `if` `__name__ ``=``=` `'__main__'``:` `    ``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 ``=` `bTreeToCList(root) ` `    ``displayCList(head) ` `    `  `# This code is contributed by PranchalK`

## C#

 `// C# Program to convert a Binary Tree ` `// to a Circular Doubly Linked List ` `using` `System;`   `// Node class represents a Node of a Tree ` `public` `class` `Node` `{` `    ``public` `int` `val;` `    ``public` `Node left, right;`   `    ``public` `Node(``int` `val)` `    ``{` `        ``this``.val = val;` `        ``left = right = ``null``;` `    ``}` `}`   `// A class to represent a tree ` `public` `class` `Tree` `{` `    ``internal` `Node root;` `    ``public` `Tree()` `    ``{` `        ``root = ``null``;` `    ``}`   `    ``// concatenate both the lists ` `    ``// and returns the head of the List ` `    ``public` `virtual` `Node concatenate(Node leftList, ` `                                    ``Node rightList)` `    ``{` `        ``// If either of the list is empty, ` `        ``// then return the other list ` `        ``if` `(leftList == ``null``)` `        ``{` `            ``return` `rightList;` `        ``}` `        ``if` `(rightList == ``null``)` `        ``{` `            ``return` `leftList;` `        ``}`   `        ``// Store the last Node of left List ` `        ``Node leftLast = leftList.left;`   `        ``// Store the last Node of right List ` `        ``Node rightLast = rightList.left;`   `        ``// Connect the last node of Left List ` `        ``// with the first Node of the right List ` `        ``leftLast.right = rightList;` `        ``rightList.left = leftLast;`   `        ``// left of first node refers to ` `        ``// the last node in the list ` `        ``leftList.left = rightLast;`   `        ``// Right of last node refers to ` `        ``// the first node of the List ` `        ``rightLast.right = leftList;`   `        ``// Return the Head of the List ` `        ``return` `leftList;` `    ``}`   `    ``// Method converts a tree to a circular ` `    ``// Link List and then returns the head ` `    ``// of the Link List ` `    ``public` `virtual` `Node bTreeToCList(Node root)` `    ``{` `        ``if` `(root == ``null``)` `        ``{` `            ``return` `null``;` `        ``}`   `        ``// Recursively convert left ` `        ``// and right subtrees ` `        ``Node left = bTreeToCList(root.left);` `        ``Node right = bTreeToCList(root.right);`   `        ``// Make a circular linked list of single ` `        ``// node (or root). To do so, make the ` `        ``// right and left pointers of this node ` `        ``// point to itself ` `        ``root.left = root.right = root;`   `        ``// Step 1 (concatenate the left list with` `        ``//          the list with single node,` `        ``//        i.e., current node) ` `        ``// Step 2 (concatenate the returned list ` `        ``//           with the right List) ` `        ``return` `concatenate(concatenate(left, root), right);` `    ``}`   `    ``// Display Circular Link List ` `    ``public` `virtual` `void` `display(Node head)` `    ``{` `        ``Console.WriteLine(``"Circular Linked List is :"``);` `        ``Node itr = head;` `        ``do` `        ``{` `            ``Console.Write(itr.val + ``" "``);` `            ``itr = itr.right;` `        ``} ``while` `(itr != head);` `        ``Console.WriteLine();` `    ``}` `}`   `// Driver Code ` `public` `class` `GFG` `{` `    ``public` `static` `void` `Main(``string``[] args)` `    ``{` `        ``// Build the tree ` `        ``Tree tree = ``new` `Tree();` `        ``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);`   `        ``// head refers to the head of the Link List ` `        ``Node head = tree.bTreeToCList(tree.root);`   `        ``// Display the Circular LinkedList ` `        ``tree.display(head);` `    ``}` `}`   `// This code is contributed by Shrikant13`

## Javascript

 ``

Output

```Circular Linked List is :
25 12 30 10 36 15
```

TIme Complexity: O(N)

As every node is visited at most once.

Auxiliary space: O(log N)

The extra space is used in recursion call stack which can grow upto a maximum size of logN as it is a binary tree.

Another approach :
The above algorithm might be difficult to understand, so here is another approach which has same time and space complexities but uses the idea of  this Convert a given Binary Tree to Doubly Linked List post. We first convert the given binary tree to a Doubly Linked List and then convert this Doubly Linked List to a Circular Linked List.

This approach was contributed by Abhijeet Kumar

Below is the implementation of the above approach:

## C++

 `// A C++ program for in-place conversion of Binary Tree to CDLL` `#include ` `using` `namespace` `std;`   `/* A binary tree node has - data , left and right pointers */` `struct` `Node` `{` `    ``int` `data;` `    ``Node* left;` `    ``Node* right;` `};` `// A utility function that converts given binary tree to ` `// a doubly linked list` `// root --> the root of the binary tree` `// head --> head of the created doubly linked list` `Node *BTree2DoublyLinkedList(Node *root, Node **head)` `{` `    ``// Base case` `    ``if` `(root == NULL) ` `      ``return` `root;`   `    ``// 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` `    ``BTree2DoublyLinkedList(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` `    ``BTree2DoublyLinkedList(root->right, head);` `    ``return` `prev;` `}` `// A simple recursive function to convert a given Binary tree to ` `// Circular Doubly Linked List using a utility function` `// root --> Root of Binary Tree` `// tail --> Pointer to tail node of created circular doubly linked list` `Node* BTree2CircularDoublyLinkedList(Node *root)` `{` `    ``Node *head = NULL;` `    ``Node *tail = BTree2DoublyLinkedList(root, &head);` `    ``// make the changes to convert a DLL to CDLL` `    ``tail->right = head;` `      ``head->left = tail;` `      ``// return the head of the created CDLL` `      ``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 circular doubly linked list */` `void` `printList(Node *head)` `{` `    ``if``(head==NULL)` `      ``return``;` `      ``Node *ptr = head;` `    ``do` `    ``{` `        ``cout << ptr->data << ``" "``;` `        ``ptr = ptr->right;` `    ``}``while``(ptr!=head);` `}`   `/* 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 = BTree2CircularDoublyLinkedList(root);`   `    ``// Print the converted list` `    ``printList(head);`   `    ``return` `0;` `}`   `// This code was contributed by Abhijeet Kumar(abhijeet19403)`

## Java

 `// A Java program for in-place conversion of Binary Tree to CDLL`   `// A binary tree node has - data, left pointer and right pointer` `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 utility recursive function to convert a given Binary tree` `    ``// to Doubly Linked List` `    ``// root --> Root of Binary Tree` `    ``void` `BTree2DoublyLinkedList(Node root)` `    ``{` `        ``// Base case` `        ``if` `(root == ``null``)` `            ``return``;`   `        ``// Recursively convert left subtree` `        ``BTree2DoublyLinkedList(root.left);`   `        ``// Now convert this node` `        ``if` `(prev == ``null``)` `            ``head = root;` `        ``else` `        ``{` `            ``root.left = prev;` `            ``prev.right = root;` `        ``}` `        ``prev = root;`   `        ``// Finally convert right subtree` `        ``BTree2DoublyLinkedList(root.right);` `    ``}` `      ``// A simple function to convert a given binary tree to Circular doubly linked list` `    ``// using a utility function` `    ``void` `BTree2CircularDoublyLinkedList(Node root)` `    ``{` `      ``BTree2DoublyLinkedList(root);` `      ``// make the changes to convert a DLL to CDLL` `      ``prev.right = head;` `      ``head.left = prev;` `    ``}`   `    ``/* Function to print nodes in a given doubly linked list */` `    ``void` `printList(Node node)` `    ``{` `      ``if``(node == ``null``)` `        ``return``;` `      ``Node curr = node; ` `        ``do` `        ``{` `            ``System.out.print(curr.data + ``" "``);` `            ``curr = curr.right;` `        ``}``while``(curr!=node);` `    ``}`   `    ``// 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.BTree2CircularDoublyLinkedList(tree.root);` `        `  `        ``// Print the converted List` `        ``tree.printList(tree.head);`   `    ``}` `}` `// This code has been contributed by Abhijeet Kumar(abhijeet19403)`

Output

`25 12 30 10 36 15 `

TIme Complexity: O(N)

As every node is visited at most once.

Auxiliary space: O(log N)

The extra space is used in recursive function call stack which can grow upto a maximum size of logN.

This article is contributed by Chirag Agarwal. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.