Z字形变换

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class Solution:
    def convert(self, s: str, numRows: int) -> str:
        if numRows==1:return s
        res = ['' for _ in range(numRows)]
        i,flag = 0,-1
        for c in s:
            if i == 0 or i==numRows-1:
                flag = -flag
            
            res[i] += c
            i += flag
        return ''.join(res)