6. Z 字形变换

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        if numRows <= 1 or numRows >= len(s):
            return s

        span, ret = (numRows - 1) * 2, []
        for i in range(0, len(s), span):
            ret.append(s[i])

        for i in range(1, numRows - 1):
            spans, direct, j = [span - 2 * i, 2 * i], 0, i
            while j < len(s):
                ret.append(s[j])
                j += spans[direct]
                direct = 1 - direct

        for i in range(numRows - 1, len(s), span):
            ret.append(s[i])

        return ''.join(ret)