Raj was to move up through a pattern of stairs of a given number **(n)**. Help him to get to the    top using the function **stairs**.
##Keep in mind :
  * If **n<1** then return ' ' .
  * There are a lot of spaces before the stair starts except for **pattern(1)**
##Examples :
  pattern(1)
          1 1
  pattern(6)
                          1 1
                      1 2 2 1  
                  1 2 3 3 2 1
              1 2 3 4 4 3 2 1
          1 2 3 4 5 5 4 3 2 1
      1 2 3 4 5 6 6 5 4 3 2 1
pattern(12)
                                                  1 1
                                              1 2 2 1
                                          1 2 3 3 2 1
                                      1 2 3 4 4 3 2 1
                                  1 2 3 4 5 5 4 3 2 1
                              1 2 3 4 5 6 6 5 4 3 2 1
                          1 2 3 4 5 6 7 7 6 5 4 3 2 1
                      1 2 3 4 5 6 7 8 8 7 6 5 4 3 2 1
                  1 2 3 4 5 6 7 8 9 9 8 7 6 5 4 3 2 1
              1 2 3 4 5 6 7 8 9 0 0 9 8 7 6 5 4 3 2 1
          1 2 3 4 5 6 7 8 9 0 1 1 0 9 8 7 6 5 4 3 2 1
      1 2 3 4 5 6 7 8 9 0 1 2 2 1 0 9 8 7 6 5 4 3 2 1
from itertools import chain

def stairs(n):
    if n < 1:
        return " "
    w = 4 * n - 1
    xs = [str(i % 10) for i in chain(range(1, n + 1), range(n, 0, -1))]
    return "\n".join(" ".join(xs[:i] + xs[-i:]).rjust(w) for i in range(1, n + 1))
from itertools import chain

def stairs(n):
    if n < 1:
        return " "
    w = 4 * n - 1
    xs = [str(i % 10) for i in chain(range(1, n + 1), range(n, 0, -1))]
    return "\n".join(" ".join(xs[:i] + xs[-i:]).rjust(w) for i in range(1, n + 1))