zigzag conversion

 1from collections import defaultdict as dd
 2
 3class Solution(object):
 4    def convert(self, s, numRows):
 5        x, y, xi, yi, i = 0, 0, 0, 0, 0
 6        o = {i:dd(list) for i in range(numRows)}
 7        while i < len(s):
 8            top = i % ((numRows-1) * 2) == 0
 9            bottom = i % (numRows-1) == 0
10            if top:
11                x, y = 0, 1
12            elif bottom:
13                x, y = 1, -1
14            o[yi][xi] = s[i]
15            xi, yi, i = xi+x, yi+y, i+1
16        return ''.join([''.join(x.values()) for x in o.values()])
17
18print(Solution().convert("PAYPALISHIRING", 3))