input: Write a function to convert a list of integers to a comma-separated string.
  assert list_to_string([1, 2, 3, 4]) == '1,2,3,4'
  assert list_to_string([10, 20, 30]) == '10,20,30'
  assert list_to_string([5, 6, 7, 8, 9]) == '5,6,7,8,9'
output: def list_to_string(lst):
      return ','.join(map(str, lst))