# 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.

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 using namespace std;    // A tree node  class node  {      public:     int data;      node *left, *right;  };     // A utility function to create // a new tree node  node *newNode(int data)  {      node *Node = new node();     Node->data = data;      Node->left = Node->right = NULL;      return(Node);  }     // Standard Inorder traversal of tree  void inorder(node *root)  {      if (root != NULL)      {          inorder(root->left);          cout << " " << 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(node *root)  {      static 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  node *fixNextPtr(node *root)  {      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;      }         // The leftmost node is head      // of linked list, return it      return (root);  }     // The main function that converts  // BST to DLL and returns head of DLL  node *BTToDLL(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(node *root)  {      while (root != NULL)      {          cout<<" "<data;          root = root->right;      }  }     // Driver code  int main(void)  {      // 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);         cout<<" Inorder Tree Traversal ";      inorder(root);         node *head = BTToDLL(root);         cout << " DLL Traversal ";      printList(head);      return 0;  }    // This code is contributed by rathbhupendra

## C

 // A simple inorder traversal based program to convert a Binary Tree to DLL #include #include    // 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(" %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;     }        // The leftmost node is head of linked list, return it     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(" %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(" Inorder Tree Traversal ");     inorder(root);        struct node *head = BTToDLL(root);        printf(" DLL Traversal ");     printList(head);     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;         }            // The leftmost node is head of linked list, return it         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);            node head = BTTtoDLL(root);            System.out.println(" DLL Traversal");         printlist(head);     } }    // 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 " %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 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        # The leftmost node is head of linked list, return it     return root     # The main function that converts BST to DLL and returns # head of DLL 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 " %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 " Inorder Tree Traversal " inorder(root)    # Static variable pre for function fixPrevPtr fixPrevPtr.pre = None head = BTToDLL(root)    print " DLL Traversal " printList(head)        # This code is contributed by Nikhil Kumar Singh(nickzuck_007)

## C#

 // C# program to convert BTT to DLL using  // simple inorder traversal  using System;    class GFG { public class node {     public int data;     public node left, right;        public node(int data)     {         this.data = data;     } }    public 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  public 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  public 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;     }        // The leftmost node is head of      // linked list, return it      return root; }    public 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  public static void printlist(node root) {     while (root != null)     {         Console.Write(root.data + " ");         root = root.right;     } }    // Standard Inorder traversal of tree  public static void inorder(node root) {     if (root == null)     {         return;     }     inorder(root.left);     Console.Write(root.data + " ");     inorder(root.right); }    public static void Main() {     // 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);        Console.WriteLine("Inorder Tree Traversal");     inorder(root);        node head = BTTtoDLL(root);        Console.WriteLine(" DLL Traversal");     printlist(head); } }    // This code is contributed by Shrikant13

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.