6. ZigZag Conversion Leetcode Python 2016 new Season

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 text, int nRows);

convert("PAYPALISHIRING", 3) should return  "PAHNAPLSIIGYIR".

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class Solution(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        if len(s) <= 1 or numRows <= 1:
            return s
        resRows = [''] * numRows
        resRowNum = 0
        step = 1
        for index in xrange(len(s)):
            resRows[resRowNum] += s[index]
            if (step == 1 and resRowNum == numRows - 1) or (step == -1 and resRowNum == 0):
                step *= -1
            resRowNum += step
        return ''.join(resRows)