Leetcode-Medium 6. ZigZag Conversion

P   A   H   N
A P L S I I G
Y   I   R

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        if numRows==1 or len(s)<=numRows:
            return s
        ans=['']*numRows
        index,step=0,1
        for x in s:
            ans[index]+=x
            if index==0:
                step=1
            elif index==numRows-1:
                step=-1
            index+=step
        return "".join(ans)