flattenBinaryTreeToList

Problem

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

Use pre-order transversal to determine seq

either use recursion or explicit stack to do the transversal

A little experiment displayed that leetcode use different test data for python and java