input: Write a function that prints out the Pascal's triangle for a given number of rows. If the given number of rows is less than 1, the function should print nothing.
  assert pascal_triangle(5)
  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(' '.join(map(str, row)))