LeetCode（Python3）6. Z字型变换

Question：

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

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

And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string s, int numRows);

Example1：

Input：s = "PAYPALISHIRING", numRows = 3

Output: "PAHNAPLSIIGYIR"

Example2: 

Input：s = "PAYPALISHIRING", numRows = 4

Output: "PINALSIGYAHRPI"

Mentality:

 这道题实质上就是一道找规律题，规律如下：

• 第 0 行：k * 2 *（numRows-1）；
• 第 n-1 行：k * 2 *（numRows-1）+（numRows-1）；
• 内部 i 行：k * 2 * (n - 1) + i  以及（k + 1） * 2 * (n - 1) - i.

Code： 

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        if numRows == 1:
            return s
        nlen = 2 * (numRows - 1)
        news = ''
        for i in range(numRows):    #逐行遍历
            j = 0
            while j + i < len(s):
                news += s[j+i]
                if i not in [0,numRows-1] and j + nlen - i < len(s):    #控制中间行
                    news += s[j+nlen-i]
                j += nlen
        return news

Result：