input: Write a python function to generate a staircase pattern with a given number of steps.
  assert staircase(3) == "  #\n ##\n###"
  assert staircase(4) == "   #\n  ##\n ###\n####"
  assert staircase(5) == "    #\n   ##\n  ###\n ####\n#####"
output: def staircase(n):
      return '\n'.join(' ' * (n - i - 1) + '#' * (i + 1) for i in range(n))