input: Write a function that prints out Pascal's triangle for a given number of rows. If the number of rows is less than 1, return None.
  
  def pascal_triangle(n):
      if n < 1:
          return None
      trow = [1]
      y = [0]
      for x in range(n):
          print(trow)
          trow = [l + r for l, r in zip(trow + y, y + trow)]
  
  pascal_triangle(5)
  
  assert pascal_triangle(5) == None
  assert pascal_triangle(0) == None
  assert pascal_triangle(-1) == None
output: if n < 1:
      return None
  trow = [1]
  y = [0]
  for x in range(n):
      print(trow)
      trow = [l + r for l, r in zip(trow + y, y + trow)]