将二叉树转化为链表

1. 将二叉树以螺旋顺序转化为双向链表

2. 将二叉树按照中序转化为链表；

http://www.cnblogs.com/xwdreamer/archive/2012/04/28/2475300.html

3. 将二叉树按照深度优先的顺序转化为链表

http://stackoverflow.com/questions/3411793/convert-a-binary-tree-to-linked-list-breadth-first-constant-storage-destructiv