题目：
思路：
怎一个秒字了得
参考：
https://leetcode-cn.com/problems/zigzag-conversion/solution/zzi-xing-bian-huan-by-jyd/
解答：
class Solution:
def convert(self, s: str, numRows: int) -> str:
if numRows < 2:
return s
res = ["" for _ in range(numRows)]
i, flag = 0, -1
for c in s:
res[i] += c
if i == 0 or i == numRows - 1: flag = -flag
i += flag
return "".join(res)
