# Convert a Binary Tree to a Circular Doubly Link List

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 List must be same as Inorder of the given Binary Tree.
• The first node of Inorder traversal must be head node of the Circular List.

Example:

## Recommended: Please solve it on “PRACTICE” first, before moving on to the solution.

The idea can be described using 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 list of single root node.
….e) Concatenate the list produced in step above (d) with rightList.

Note that the above code traverses tree in Postorder fashion. We can traverse in inorder fashion also. We can first concatenate left subtree and root, then recur for 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 above idea.

## C++

```// C++ Program to convert a Binary Tree
// to a Circular Doubly Linked List
#include<iostream>
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
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);
}

{
cout << "Circular Linked List is :\n";
do
{
cout << itr->data <<" ";
itr = itr->right;
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);

return 0;
}
```

## Java

```// Java Program to convert a Binary Tree to a

// 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
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);
}

{
do
{
System.out.print(itr.val+ " " );
itr = itr.right;
}
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);

}
}
```

Output:

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

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

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