title: converting a binary search tree to doubly linked list
body: <p>This question was asked in a recent coding interview.    </p>
  
  <p><strong>Q :</strong> Given a binary tree, write a program to convert it to a doubly linked list. The nodes in the doubly linked list are arranged in a sequence formed by a zig-zag level order traversal</p>
  
  <p><strong>My approach</strong></p>
  
  <p>i could always do the zig-zag level order traversal of the tree and store it in an array 
  an then make a double linked list.
  but the question demands for a in-place solution.
  can anyone help in explaining the recursive approach should be used?</p>