Convert a Binary Tree into Doubly Linked List in spiral fashion

Given a Binary Tree, convert it into a Doubly Linked List where the nodes are represented Spirally. The left pointer of the binary tree node should act as a previous node for created DLL and the right pointer should act as the next node.

The solution should not allocate extra memory for DLL nodes. It should use binary tree nodes for creating DLL i.e. only change of pointers is allowed.

We can do this by doing a spiral order traversal in O(n) time and O(n) extra space. The idea is to use deque (Double-ended queue) that can be expanded or contracted on both ends (either its front or it’s back). 

We do something similar to level order traversal but to maintain spiral order, for every odd level, we dequeue node from the front and insert its left and right children in the back of the deque data structure. 

And for each even level, we dequeue node from the back and insert its right and left children in the front of the deque. We also maintain a stack to store Binary Tree nodes. Whenever we pop nodes from deque, we push that node into the stack.

Later, we pop all nodes from the stack and push the nodes at the beginning of the list. We can avoid the use of stack if we maintain a tail pointer that always points to the last node of DLL and inserts nodes in O(1) time in the end.

Convert a Binary Tree into Doubly Linked List in spiral fashion : https://www.geeksforgeeks.org/convert-a-binary-tree-into-doubly-linked-list-in-spiral-fashion/