input: Write a python function to parse a string of comma-separated integers and return a list of integers.
  assert parse_string("1,2,3") == [1, 2, 3]
  assert parse_string("10,20,30,40") == [10, 20, 30, 40]
  assert parse_string("5,15,25") == [5, 15, 25]
output: def parse_string(s):
      return [int(x) for x in s.split(',')]