input: Write a function to print Pascal's triangle for a given number of rows. If the input number is less than 1, do not print 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
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)]