LeetCode Q114: 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

Solution:
Using recursion, every time, link the right sub-tree to the deepest leaf node of the left sub-tree.

My code accepted in one pass, first time~!! Woo-Hoo~