Create Flatten Binary Tree to Linked List

Given a binary tree, flatten it to a linked list in-place.
For example,
Given
         1
        / \
       2   5
      / \   \
     3   4   6


The flattened tree should look like:
   1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6