not a usable version, only the insert() and operator[] can use in a simple
way, it is just for showing the sketch of my original design, and showing
the problem I faced and the lack of knowledge.
I want to use the bad code as an intial commit to record how weak I am
now, so that I can improve step by step as the code does.
solve the problem of namespace pollution,
add include guard
destructor is available
change the structure of codes, imitating rb_tree and map in STL
ignore tags....
delete tags in index
change the folder structure of codes, and also the design of trie, but
codes are with compilation error