Flatten Binary Tree to Linked List

Problem:

Given a binary tree, flatten it to a linked list in-place using preorder tree traversal.

Examples:

					
Given
         1
        / \
       2   5
      / \   \
     3   4   6


The flattened tree should look like:
  1
    \
     2
      \
       3
        \
         4
          \
           5
            \
             6
				

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