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

02 Dec, 2021
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++
Java
Python3
C#
Javascript
// 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;}
// 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)
# 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 = None  class 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.head  def 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)
// 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
<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.
The below code is the intuitive iterative implementation of the above inorder traversal approach (inspired from this video) –
C++
Node * bToDLL(Node *root){    stack<pair<Node*, int>> s;    s.push({root, 0});    vector<int> res;    bool flag = true;    Node* head = NULL;    Node* prev = NULL;    while(!s.empty()) {        auto x = s.top();        Node* t = x.first;        int state = x.second;        s.pop();        if(state == 3 or t == NULL) continue;        s.push({t, state+1});        if(state == 0) s.push({t->left, 0});        else if(state == 1) {            if(prev) prev->right = t;            t->left = prev;            prev = t;            if(flag) {                head = t;                flag = false;            }        }        else if(state == 2) s.push({t->right, 0});    }    return head;}
Time complexity: O(N)
Space complexity: O(N)
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
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thecelestial_25
Accolite
Amazon
doubly linked list
Goldman Sachs
Microsoft
Morgan Stanley
Snapdeal
Linked List
Tree
Morgan Stanley
Accolite
Amazon
Microsoft
Snapdeal
Goldman Sachs
Linked List
Tree
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