【leetcode刷题】20T5-Z 字形变换

L   C   I   R
E T O E S I I G
E   D   H   N

示例 1:
输入: s = "LEETCODEISHIRING", numRows = 3
输出: "LCIRETOESIIGEDHN"

示例 2:
输入: s = "LEETCODEISHIRING", numRows = 4
输出: "LDREOEIIECIHNTSG"
解释:
L     D     R
E   O E   I I
E C   I H   N
T     S     G

class Solution(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        # 特殊情况
        if numRows == 1 or numRows >= len(s):
            return s

        res = [[] for i in range(numRows)]
        ascend = True
        index = 0
        for i, si in enumerate(s):
            res[index].append(si)
            # 到达0或者len(res)-1，考虑换方向，除了第一次
            if i != 0 and (index == 0 or index == len(res) - 1):
                ascend = not ascend
            if ascend:
                index += 1
            else:
                index -= 1
        return ''.join(''.join(ri) for ri in res)