exercise 2-5
exercise 2-6
exercise 2-7
exercise 2-8
exercise 2-9
exercise 2-10
exercise 1-1
exercise 1-2
exercise 1-3
exercise 1-4
exercise 1-5
exercise 1-6
exercise 1-7
exercise 1-8
exercise 1-9
exercise 1-10
exercise 1-11
exercise 1-12
change the dir tree