input: Write a function to print out Pascal's triangle for a given number of rows. The function should take an integer `n` as input and print each row of the Pascal's triangle up to `n` rows. If `n` is less than 1, the function should not print anything.
  assert pascal_triangle(5)
  '''
  [1]
  [1, 1]
  [1, 2, 1]
  [1, 3, 3, 1]
  [1, 4, 6, 4, 1]
  '''
  assert pascal_triangle(0)==None
  assert pascal_triangle(-1)==None
output: def pascal_triangle(n):
      if n < 1:
          return
      triangle = [[1]]
      for i in range(1, n):
          row = [1]
          for j in range(1, i):
              row.append(triangle[i-1][j-1] + triangle[i-1][j])
          row.append(1)
          triangle.append(row)
      for row in triangle:
          print(row)