input: Write a function to print out Pascal's triangle for a given number of rows. If the input number is less than 1, do nothing.
  
  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(5)==None
  assert pascal_triangle(0)==None
  assert pascal_triangle(-3)==None
  assert pascal_triangle(3)==None
output: 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)]