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
