Flatten Binary Tree to Linked List (Java)

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

Solution:

Divide and Conquer, convert left and right tree to list separately, then connect root. right to list converted by left tree, make root. left to null, then search for the lastNode of right tree and connect it to list converted by original right tree.