Z字形变换

将字符串 "PAYPALISHIRING" 以Z字形排列成给定的行数：

P   A   H   N
A P L S I I G
Y   I   R

之后从左往右，逐行读取字符："PAHNAPLSIIGYIR"

实现一个将字符串进行指定行数变换的函数:

string convert(string s, int numRows);

示例 1:

输入: s = "PAYPALISHIRING", numRows = 3
输出: "PAHNAPLSIIGYIR"

示例 2:

输入: s = "PAYPALISHIRING", numRows = 4
输出: "PINALSIGYAHRPI"
解释:

P     I    N
A   L S  I G
Y A   H R
P     I

class Solution(object):
    def convert(self,s,numRows):
        '''

        :type s:str
        :type numRows:int
        :rtype:str
        '''
        if numRows==1 or numRows >= len(s):
            return s
        L=['']*numRows #建立长度为num的空列表
        index,step=0,1
        for x in s:
            L[index] += x
            if index == 0:
                step=1
            elif index==numRows-1:
                step=-1
            index += step
        return ''.join(L)

s='PAYPALISHIRING'
str=Solution.convert(1,s,3)
print(str)