1. First commit of tree
	- now implement the insert and print
2. TODO:
	- refine the print
	- add the roate/remove/find interface
1. Fix bug in insert()
2. Rename the case
1. Implement the get_balance_factor()
	- Use the tree-travesal
	- The call times of get_max_depth_()
	- Big O is the same with pre-order tree-travesal, log(N)

2. Use a simple way to ouput the b-tree
	- I just can't print the hierarchy of the tree in terminal. :-P
	- I mean the below. It's too complicated to compute the indent/width of each node.

						root
				l1               r1
			 l2    r2         l3   r3
						   l4
							  r5
1. Add the check_balance and roate oeprations when perform insert
	- This will ensure the tree will always be balance after any modify
2. Remove some useless interface
	- print_all()
	- print_node_()
1. Implement the find and remove interface
2. Add test case for these two interface
3. TODO: the reset interface. which was used to change the value of one node

The b_tree is almost finished. hoho~