4.1 HW 1
 4.2 HW 2
 4.3 HW 3
 4.4 HW 3 different solution
 4.5 HW 4
 4.6 HW 5
 4.7 HW 6
 4.8 HW 7
 4.9 HW 8
 4.10 HW 9
 4.11 HW 10
 4.12 HW 11