Convert Binary Tree to Doubly Linked List by fixing left and right pointers (original) (raw)
Last Updated : 23 Jul, 2025
Given a **Binary Tree, the task is to convert it to a **Doubly Linked List (DLL) in place. The **left and **right pointers in **nodes will be used as **previous and **next pointers respectively in converted DLL. The order of nodes in **DLL must be the same as the order of the given **Binary Tree. The first node of **Inorder traversal (the **leftmost node in Binary Tree) must be the head node of the DLL.
**Examples:
**Input:
**Output:
**Explanation: _The above binary tree is converted into doubly linked list where left pointer of the binary tree node act as the previous node and right pointer of the binary tree node act as the next node.
**Input:
**Output:
**Explanation: _The above binary tree is converted into doubly linked list where left pointer of the binary tree node act as the previous node and right pointer of the binary tree node act as the next node.
**Approach:
The idea involves converting a Binary Tree to a Doubly Linked List (DLL) in place by first fixing the left pointers** during an in-order traversal to point to the **previous node, then fixing the right pointers by traversing the modified tree in **reverse using the left pointers to point to the next node in the DLL.
Step by step approach:
- Perform an **in-order traversal of the tree to fix the **left pointers, updating each node’s left to point to the **previously visited node.
- Use the rightmost node ****(last node in DLL)** and **traverse back using left pointers to fix the right pointers for each node.
- Ensure the **leftmost node ****(first in DLL**) is returned as the **head of the DLL. C++ `
// C++ program for in-place // conversion of Binary Tree to DLL #include <bits/stdc++.h> using namespace std;
class Node { public: int data; Node* left; Node* right; Node (int x) { data = x; left = nullptr; right = nullptr; } };
// function to fix left pointers void fixPrevPtr(Node* root, Node*& prev) { if (!root) return;
// Recur for left subtree
fixPrevPtr(root->left, prev);
// Update left pointer
root->left = prev;
// Update previous pointer
prev = root;
// Recur for right subtree
fixPrevPtr(root->right, prev);}
// function to fix right pointers and get the head Node* fixNextPtr(Node* root) {
// Find the rightmost node
Node* prev = nullptr;
while (root && root->right) root = root->right;
// Traverse back using left
// pointers to fix right pointers
while (root) {
root->right = prev;
prev = root;
root = root->left;
}
// Return the head (leftmost node)
return prev;}
// The main function that converts // Binary Tree to DLL and returns head of DLL Node* bToDLL(Node* root) { if (!root) return nullptr;
Node* prev = nullptr;
// Fix the left pointers
fixPrevPtr(root, prev);
// Fix the right pointers
// and return the head
return fixNextPtr(root);}
void printList(Node* head) { Node* curr = head;
while (curr != nullptr) {
cout << curr->data << " ";
curr = curr->right;
}
cout << endl;}
int main() {
// Create a hard coded binary tree
// 10
// / \
// 12 15
// / \ /
// 25 30 36
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);
Node* head = bToDLL(root);
printList(head);
return 0;}
C
// C program for in-place // conversion of Binary Tree to DLL #include <stdio.h> #include <stdlib.h>
struct Node { int data; struct Node* left; struct Node* right; };
// function to fix left pointers void fixPrevPtr(struct Node* root, struct Node** prev) { if (!root) return;
// Recur for left subtree
fixPrevPtr(root->left, prev);
// Update left pointer
root->left = *prev;
// Update previous pointer
*prev = root;
// Recur for right subtree
fixPrevPtr(root->right, prev);}
// function to fix right pointers and get the head struct Node* fixNextPtr(struct Node* root) {
// Find the rightmost node
struct Node* prev = NULL;
while (root && root->right) root = root->right;
// Traverse back using left
// pointers to fix right pointers
while (root) {
root->right = prev;
prev = root;
root = root->left;
}
// Return the head (leftmost node)
return prev;}
// The main function that converts // Binary Tree to DLL and returns head of DLL struct Node* bToDLL(struct Node* root) { if (!root) return NULL;
struct Node* prev = NULL;
// Fix the left pointers
fixPrevPtr(root, &prev);
// Fix the right pointers
// and return the head
return fixNextPtr(root);}
void printList(struct Node* head) { struct Node* curr = head;
while (curr != NULL) {
printf("%d ", curr->data);
curr = curr->right;
}
printf("\n");}
struct Node* createNode(int x) { struct Node* newNode = (struct Node*)malloc(sizeof(struct Node)); newNode->data = x; newNode->left = NULL; newNode->right = NULL; return newNode; }
int main() {
// Create a hard coded binary tree
// 10
// / \
// 12 15
// / \ /
// 25 30 36
struct Node* root = createNode(10);
root->left = createNode(12);
root->right = createNode(15);
root->left->left = createNode(25);
root->left->right = createNode(30);
root->right->left = createNode(36);
struct Node* head = bToDLL(root);
printList(head);
return 0;}
Java
// Java program for in-place // conversion of Binary Tree to DLL import java.util.ArrayList;
class Node { int data; Node left; Node right;
Node(int x) {
data = x;
left = null;
right = null;
}}
class GfG {
// function to fix left pointers
static void fixPrevPtr(Node root, Node[] prev) {
if (root == null) return;
// Recur for left subtree
fixPrevPtr(root.left, prev);
// Update left pointer
root.left = prev[0];
// Update previous pointer
prev[0] = root;
// Recur for right subtree
fixPrevPtr(root.right, prev);
}
// function to fix right pointers and get the head
static Node fixNextPtr(Node root) {
// Find the rightmost node
Node prev = null;
while (root != null && root.right != null) root = root.right;
// Traverse back using left
// pointers to fix right pointers
while (root != null) {
root.right = prev;
prev = root;
root = root.left;
}
// Return the head (leftmost node)
return prev;
}
// The main function that converts
// Binary Tree to DLL and returns head of DLL
static Node bToDLL(Node root) {
if (root == null) return null;
Node[] prev = new Node[1];
// Fix the left pointers
fixPrevPtr(root, prev);
// Fix the right pointers
// and return the head
return fixNextPtr(root);
}
static void printList(Node head) {
Node curr = head;
while (curr != null) {
System.out.print(curr.data + " ");
curr = curr.right;
}
System.out.println();
}
public static void main(String[] args) {
// Create a hard coded binary tree
// 10
// / \
// 12 15
// / \ /
// 25 30 36
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);
Node head = bToDLL(root);
printList(head);
}}
Python
Python program for in-place
conversion of Binary Tree to DLL
class Node: def init(self, x): self.data = x self.left = None self.right = None
function to fix left pointers
def fixPrevPtr(root, prev): if not root: return
# Recur for left subtree
fixPrevPtr(root.left, prev)
# Update left pointer
root.left = prev[0]
# Update previous pointer
prev[0] = root
# Recur for right subtree
fixPrevPtr(root.right, prev)function to fix right pointers and get the head
def fixNextPtr(root):
# Find the rightmost node
prev = None
while root and root.right:
root = root.right
# Traverse back using left
# pointers to fix right pointers
while root:
root.right = prev
prev = root
root = root.left
# Return the head (leftmost node)
return prevThe main function that converts
Binary Tree to DLL and returns head of DLL
def bToDLL(root): if not root: return None
prev = [None]
# Fix the left pointers
fixPrevPtr(root, prev)
# Fix the right pointers
# and return the head
return fixNextPtr(root)def printList(head): curr = head while curr: print(curr.data, end=" ") curr = curr.right print()
if name == "main":
# Create a hard coded binary tree
# 10
# / \
# 12 15
# / \ /
# 25 30 36
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)
head = bToDLL(root)
printList(head)C#
// C# program for in-place // conversion of Binary Tree to DLL using System; using System.Collections.Generic;
class Node { public int data; public Node left, right;
public Node(int x) {
data = x;
left = null;
right = null;
}}
class GfG {
// function to fix left pointers
static void fixPrevPtr(Node root, ref Node prev) {
if (root == null) return;
// Recur for left subtree
fixPrevPtr(root.left, ref prev);
// Update left pointer
root.left = prev;
// Update previous pointer
prev = root;
// Recur for right subtree
fixPrevPtr(root.right, ref prev);
}
// function to fix right pointers and get the head
static Node fixNextPtr(Node root) {
// Find the rightmost node
Node prev = null;
while (root != null && root.right != null) root = root.right;
// Traverse back using left
// pointers to fix right pointers
while (root != null) {
root.right = prev;
prev = root;
root = root.left;
}
// Return the head (leftmost node)
return prev;
}
// The main function that converts
// Binary Tree to DLL and returns head of DLL
static Node bToDLL(Node root) {
if (root == null) return null;
Node prev = null;
// Fix the left pointers
fixPrevPtr(root, ref prev);
// Fix the right pointers
// and return the head
return fixNextPtr(root);
}
static void printList(Node head) {
Node curr = head;
while (curr != null) {
Console.Write(curr.data + " ");
curr = curr.right;
}
Console.WriteLine();
}
static void Main(string[] args) {
// Create a hard coded binary tree
// 10
// / \
// 12 15
// / \ /
// 25 30 36
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);
Node head = bToDLL(root);
printList(head);
}}
JavaScript
// JavaScript program for in-place // conversion of Binary Tree to DLL
class Node { constructor(x) { this.data = x; this.left = null; this.right = null; } }
// function to fix left pointers function fixPrevPtr(root, prev) { if (!root) return;
// Recur for left subtree
fixPrevPtr(root.left, prev);
// Update left pointer
root.left = prev.node;
// Update previous pointer
prev.node = root;
// Recur for right subtree
fixPrevPtr(root.right, prev);}
// function to fix right pointers and get the head function fixNextPtr(root) {
// Find the rightmost node
let prev = null;
while (root && root.right) root = root.right;
// Traverse back using left
// pointers to fix right pointers
while (root) {
root.right = prev;
prev = root;
root = root.left;
}
// Return the head (leftmost node)
return prev;}
// The main function that converts // Binary Tree to DLL and returns head of DLL function bToDLL(root) { if (!root) return null;
let prev = { node: null };
// Fix the left pointers
fixPrevPtr(root, prev);
// Fix the right pointers
// and return the head
return fixNextPtr(root);}
function printList(head) { let curr = head;
while (curr) {
console.log(curr.data + " ");
curr = curr.right;
}
console.log();}
// Create a hard coded binary tree
// 10
// /
// 12 15
// / \ /
// 25 30 36
let 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);
let head = bToDLL(root);
printList(head);
`
**Time Complexity: O(n), where n is the number of nodes in Binary tree.
**Auxiliary Space: O(n)
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