Java: Convert given Binary Tree to Doubly Linked List in Linear Time

Difficulty Level: HARD

Problem Statement:

Given a Binary Tree. Convert it into a Doubly linked list. 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 the Inorder of the given Binary Tree. The first node of Inorder traversal (leftmost node in BT) must be the head node of the DLL.

Input:

Output:

Extracted Double Linked List is: 
11 15 32 12 28 23