clear old codes
	deleted:    2.1/Ex2.1_2.cpp
	deleted:    2.1/Ex2.1_4.cpp
	deleted:    2.3/2.3-7.cpp
	deleted:    2.3/ex2.3-2.cpp
	deleted:    2.3/ex2.3-4.cpp
	deleted:    2.3/ex2.3-5.cpp