input: from typing import List
  
  def pascal_triangle(n: int) -> List[List[int]]:
      """Prints out Pascal's triangle for a given number of rows.
  
      >>> pascal_triangle(0)
      []
      >>> pascal_triangle(1)
      [[1]]
      >>> pascal_triangle(5)
      [[1], [1, 1], [1, 2, 1], [1, 3, 3, 1], [1, 4, 6, 4, 1]]
      """
      if n <= 0:
          return []
      triangle = []
      trow = [1]
      y = [0]
      for _ in range(n):
          triangle.append(trow)
          trow = [l + r for l, r in zip(trow + y, y + trow)]
      return triangle
output:     if n <= 0:
          return []
      triangle = []
      trow = [1]
      y = [0]
      for _ in range(n):
          triangle.append(trow)
          trow = [l + r for l, r in zip(trow + y, y + trow)]
      return triangle