input: Write a function to print out the Pascal's triangle for a given number of rows. If the given number of rows is 0 or negative, do not print any rows.
  
  def pascal_triangle(n): 
      if n <= 0:
          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(1)==None
  assert pascal_triangle(3)==None
output: def pascal_triangle(n): 
      if n <= 0:
          return
      trow = [1] 
      y = [0] 
      for x in range(n): 
          print(trow) 
          trow=[l+r for l,r in zip(trow + y, y + trow)]