Convert a given Binary Tree to Doubly Linked List | Set 3
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 the same as in Inorder for the given Binary Tree. The first node of Inorder traversal (leftmost node in BT) must be the head node of the DLL.
The 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 in order 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 to the 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.
The 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 listvoid 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 pointersclass 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) |
Python3
# Python program for conversion of# binary tree to doubly linked list.class Node: def __init__(self, val): self.right = None self.data = val self.left = Noneclass BtoDll: def __init__(self): self.head = None self.tail = None def convert(self, root): # Base case if root is None: return # Recursively convert left subtree self.convert(root.left) # Now convert this node node = root if self.head is None: self.head = node else: self.tail.right = node node.left = self.tail self.tail = node # Finally convert right subtree self.convert(root.right) return self.headdef BinaryTree2DoubleLinkedList(root): ''' A simple recursive function to convert a given Binary tree to Doubly Linked List root --> Root of Binary Tree ''' converter = BtoDll() return converter.convert(root)def print_dll(head): '''Function to print nodes in given doubly linked list''' while head is not None: print(head.data, end=" ") head = head.right# Driver program to test above functionsif __name__ == "__main__": # Let us create the tree as # shown in above diagram 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) # convert to DLL head = BinaryTree2DoubleLinkedList(root) # Print the converted list print_dll(head) # This code is contributed by codesankalp (SANKALP) |
C#
// A C# program for in-place conversion// of Binary Tree to DLLusing System;// A binary tree node has data, left// pointers and right pointerspublic 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 listpublic Node head;// Initialize previously visited node// as NULL. This is static so that the// same value is accessible in all// recursive callspublic static Node prev = null;// A simple recursive function to// convert a given Binary tree// to Doubly Linked List// root --> Root of Binary Treepublic 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 Codepublic 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 |
Javascript
<script> // A javascript program for in-place conversion of Binary Tree to DLL // A binary tree node has data, left pointers and right pointers class Node { constructor(val) { this.data = val; this.left = null; this.right = null; } } var root; // head --> Pointer to head node of created doubly linked list var head; // Initialize previously visited node as NULL. This is // so that the same value is accessible in all recursive // calls var prev = null; // A simple recursive function to convert a given Binary tree // to Doubly Linked List // root --> Root of Binary Tree function BinaryTree2DoubleLinkedList(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 */ function printList(node) { while (node != null) { document.write(node.data + " "); node = node.right; } } // Driver program to test above functions // Let us create the tree as shown in above diagram 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); // convert to DLL BinaryTree2DoubleLinkedList(root); // Print the converted List printList(head); // This code contributed by umadevi9616</script> |
Output:
25 12 30 10 36 15
Note that the use of static variables like above is not a recommended practice (we have used static for simplicity). Imagine a situation where the same function is called for two or more trees. The old value of prev would be used in the next call for a different tree. To avoid such problems, we can use a 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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