Binary_Tree_to_DLL.cpp

/*
Problem Statement:
-----------------
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 (leftmost node in BT) must be the head node of the DLL.

Example 1:
--------
Input:
      1
    /  \
   3    2
Output:
3 1 2 
2 1 3 

Explanation: DLL would be 3<=>1<=>2

Example 2:
---------
Input:
       10
      /   \
     20   30
   /   \
  40   60
Output:
40 20 60 10 30 
30 10 60 20 40

Explanation:  DLL would be 40<=>20<=>60<=>10<=>30.

Your Task: You don't have to take input. Complete the function bToDLL() that takes root node of the tree as a parameter 
and returns the head of DLL . The driver code prints the DLL both ways.

Expected Time Complexity: O(N).
Expected Auxiliary Space: O(H).
Note: H is the height of the tree and this space is used implicitly for recursion stack.
*/

// Link --> https://practice.geeksforgeeks.org/problems/binary-tree-to-dll/1

// Code:
class Solution
{
    public: 
    Node *previous = NULL;
    Node *head = NULL;
    Node* bToDLL(Node *root)
    {
        if(root == NULL)
            return NULL;
        
        bToDLL(root->left);
        
        if(previous == NULL)
        {
            head = root;
            previous = root;
        }
        else
        {
            previous->right = root;
            root->left = previous;
            previous = root;
        }
        
        bToDLL(root->right);
        
        return head;
    }
};