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