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