Uniform Iterative&Recursive solutions for tree traversal in in/pre/post order

https://github.com/tianlixu/leetcode/blob/master/94-binary-tree-inorder-traversal.cpp
https://github.com/tianlixu/leetcode/blob/master/145-binary-tree-preorder-traversal.cpp
https://github.com/tianlixu/leetcode/blob/master/146-binary-tree-postorder-traversal.cpp