Convert a given Binary Tree to Doubly Linked List

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

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

In this post, a third solution is discussed which seems to be the simplest of all. The idea is to do inorder 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 of prev and previous of this node as prev.

Thanks to rahul, wishall and all other readers for their useful comments on the above two posts.

Following is the implementation of this solution.

C++

// A C++ program for in-place conversion of Binary Tree to DLL 
#include <iostream> 
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 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; 
} 

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) 

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 

Output:

25 12 30 10 36 15

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

Time Complexity: The above program does a simple inorder traversal, so time complexity is O(n) where n is the number of nodes in given binary tree.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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My Personal Notes

Convert a given Binary Tree to Doubly Linked List | Set 3

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

C++

Java

C#

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