input: Write a function to check if a list contains duplicates.
  assert contains_duplicates([1, 2, 3, 4, 5, 6])==False
  assert contains_duplicates([1, 2, 3, 4, 5, 6, 6])==True
  assert contains_duplicates([1, 1, 2, 3, 4, 5])==True
output: def contains_duplicates(lst):
      return len(lst) != len(set(lst))