input: Write a function to print Pascal's triangle for a given number of rows. If the number of rows is less than 1, the function should return without printing anything.
  
  def pascal_triangle(n):
      if n < 1:
          return
      trow = [1]
      y = [0]
      for x in range(n):
          print(trow)
          trow = [l + r for l, r in zip(trow + y, y + trow)]
  
  assert pascal_triangle(0) == None
  assert pascal_triangle(1) == None
  assert pascal_triangle(5) == None
  assert pascal_triangle(3) == None
output: def pascal_triangle(n):
      if n < 1:
          return
      trow = [1]
      y = [0]
      for x in range(n):
          print(trow)
          trow = [l + r for l, r in zip(trow + y, y + trow)]