6. Z 字形变换

思路1:72 ms 13 MB。考虑给字符串进行编号。对于“LEETCODEISHIRING”，当行数为4时，编号为012321，不断重复，最后根据编号（实际是行号）进行稳定排序，最终即答案。

import copy

class Solution():
    def convert(self, s , numRows ):
        str = [x for x in s]
        index1 = [x for x in range(0,numRows)]
        index2 = [x for x in range(numRows-2,0,-1)]
        index1+=index2
        basicindex = copy.copy(index1) # 浅赋值 基本的编号，后续重复
        for i in range(len(str)//len(index1)+1):
            index1+=basicindex
        str = list(zip(index1,str))
        newString = sorted(str,key=lambda x:x[0])
        finalString = [x[1] for x in newString]
        ans = "".join(finalString)
        return ans

思路2：56 ms 13.1 MB。上面的排序过程还是比较费时。基于上面的思想，直接在遍历字符串的过程中，在对应的行加上字符即可。

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)