*[?]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

Thoughts

Go down through the left, when right is not null, push right to stack.

reference: 

http://gongxuns.blogspot.com/2012/12/leetcode-flatten-binary-tree-to-linked.html

*[?]Flatten Binary Tree to Linked List

原文：http://www.cnblogs.com/hygeia/p/4774758.html