Leetcode: ZigZag Conversion @Python

class Solution:
    # @return a string
    def convert(self, s, nRows):
        if nRows==1: return s
        tmp=['' for i in range(nRows)]
        index=-1; step=1
        for i in range(len(s)):
            index+=step
            if index==nRows:
                index-=2; step=-1
            elif index==-1:
                index=1; step=1
            tmp[index]+=s[i]
        return ''.join(tmp)