# Convert a given Binary Tree to Doubly Linked List | Set 2

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 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.

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

A solution to this problem is discussed in this post.
In this post, another simple and efficient solution is discussed. The solution discussed here has two simple steps.

1) Fix Left Pointers: In this step, we change left pointers to point to previous nodes in DLL. The idea is simple, we do inorder traversal of tree. In inorder traversal, we keep track of previous visited node and change left pointer to the previous node. See fixPrevPtr() in below implementation.

2) Fix Right Pointers: The above is intuitive and simple. How to change right pointers to point to next node in DLL? The idea is to use left pointers fixed in step 1. We start from the rightmost node in Binary Tree (BT). The rightmost node is the last node in DLL. Since left pointers are changed to point to previous node in DLL, we can linearly traverse the complete DLL using these pointers. The traversal would be from last to first node. While traversing the DLL, we keep track of the previously visited node and change the right pointer to the previous node. See fixNextPtr() in below implementation.

## C

// A simple inorder traversal based program to convert a Binary Tree to DLL
#include<stdio.h>
#include<stdlib.h>

// A tree node
struct node
{
int data;
struct node *left, *right;
};

// A utility function to create a new tree node
struct node *newNode(int data)
{
struct node *node = (struct node *)malloc(sizeof(struct node));
node->data = data;
node->left = node->right = NULL;
return(node);
}

// Standard Inorder traversal of tree
void inorder(struct node *root)
{
if (root != NULL)
{
inorder(root->left);
printf("\t%d",root->data);
inorder(root->right);
}
}

// Changes left pointers to work as previous pointers in converted DLL
// The function simply does inorder traversal of Binary Tree and updates
// left pointer using previously visited node
void fixPrevPtr(struct node *root)
{
static struct node *pre = NULL;

if (root != NULL)
{
fixPrevPtr(root->left);
root->left = pre;
pre = root;
fixPrevPtr(root->right);
}
}

// Changes right pointers to work as next pointers in converted DLL
struct node *fixNextPtr(struct node *root)
{
struct node *prev = NULL;

// Find the right most node in BT or last node in DLL
while (root && root->right != NULL)
root = root->right;

// Start from the rightmost node, traverse back using left pointers.
// While traversing, change right pointer of nodes.
while (root && root->left != NULL)
{
prev = root;
root = root->left;
root->right = prev;
}

return (root);
}

// The main function that converts BST to DLL and returns head of DLL
struct node *BTToDLL(struct node *root)
{
// Set the previous pointer
fixPrevPtr(root);

// Set the next pointer and return head of DLL
return fixNextPtr(root);
}

// Traverses the DLL from left tor right
void printList(struct node *root)
{
while (root != NULL)
{
printf("\t%d", root->data);
root = root->right;
}
}

// Driver program to test above functions
int main(void)
{
// Let us create the tree shown in above diagram
struct 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);

printf("\n\t\tInorder Tree Traversal\n\n");
inorder(root);

printf("\n\n\t\tDLL Traversal\n\n");
return 0;
}

## Java

// Java program to convert BTT to DLL using
// simple inorder traversal

public class BinaryTreeToDLL
{
static class node
{
int data;
node left, right;

public node(int data)
{
this.data = data;
}
}

static node prev;

// Changes left pointers to work as previous
// pointers in converted DLL The function
// simply does inorder traversal of Binary
// Tree and updates left pointer using
// previously visited node
static void fixPrevptr(node root)
{
if (root == null)
return;

fixPrevptr(root.left);
root.left = prev;
prev = root;
fixPrevptr(root.right);

}

// Changes right pointers to work
// as next pointers in converted DLL
static node fixNextptr(node root)
{
// Find the right most node in
// BT or last node in DLL
while (root.right != null)
root = root.right;

// Start from the rightmost node, traverse
// back using left pointers. While traversing,
// change right pointer of nodes
while (root != null && root.left != null)
{
node left = root.left;
left.right = root;
root = root.left;
}

return root;
}

static node BTTtoDLL(node root)
{
prev = null;

// Set the previous pointer
fixPrevptr(root);

// Set the next pointer and return head of DLL
return fixNextptr(root);
}

// Traverses the DLL from left tor right
static void printlist(node root)
{
while (root != null)
{
System.out.print(root.data + " ");
root = root.right;
}
}

// Standard Inorder traversal of tree
static void inorder(node root)
{
if (root == null)
return;
inorder(root.left);
System.out.print(root.data + " ");
inorder(root.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);

System.out.println("Inorder Tree Traversal");
inorder(root);

System.out.println("\nDLL Traversal");
}
}

// This code is contributed by Rishabh Mahrsee

## Python

# A simple inorder traversal based program to convert a
# Binary Tree to DLL

# A Binary Tree node
class Node:

# Constructor to create a new tree node
def __init__(self, data):
self.data = data
self.left = None
self.right = None

# Standard Inorder traversal of tree
def inorder(root):

if root is not None:
inorder(root.left)
print "\t%d" %(root.data),
inorder(root.right)

# Changes left pointers to work as previous pointers
# in converted DLL
# The function simply does inorder traversal of
# left pointer using previously visited node
def fixPrevPtr(root):
if root is not None:
fixPrevPtr(root.left)
root.left = fixPrevPtr.pre
fixPrevPtr.pre = root
fixPrevPtr(root.right)

# Changes right pointers to work as nexr pointers in
# converted DLL
def fixNextPtr(root):

prev = None
# Find the right most node in BT or last node in DLL
while(root and root.right != None):
root = root.right

# Start from the rightmost node, traverse back using
# left pointers
# While traversing, change right pointer of nodes
while(root and root.left != None):
prev = root
root = root.left
root.right = prev

return root

# The main function that converts BST to DLL and returns
def BTToDLL(root):

# Set the previous pointer
fixPrevPtr(root)

# Set the next pointer and return head of DLL
return fixNextPtr(root)

# Traversses the DLL from left to right
def printList(root):
while(root != None):
print "\t%d" %(root.data),
root = root.right

# Driver program to test above function
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)

print "\n\t\t Inorder Tree Traversal\n"
inorder(root)

# Static variable pre for function fixPrevPtr
fixPrevPtr.pre = None

print "\n\n\t\tDLL Traversal\n"

# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

Output:
Inorder Tree Traversal

25      12      30      10      36      15

DLL Traversal

25      12      30      10      36      15

Time Complexity: O(n) where n is the number of nodes in given Binary Tree. The solution simply does two traversals of all Binary Tree nodes.

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