empty to nil
pass given 1 return empty list test case
add second test case: given 2 renturn {2}
pass second test case
pass 3rd testcase: given 3 return {3}
add 4th test case: given 4 return {2, 2}
pass 5th test case: given 5 and return {5}
remove given 5 return {5} test case
add 5th test case: given 6 return {2, 3}
finish