[LeetCode #6] ZigZag Conversion

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".

class Solution(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        
        if numRows < 2: return s
        
        ret = ""        
        inc = 2 * (numRows - 1)
        
        for i in range(numRows):
            j = i
            d1 = (numRows - i  -1) * 2
            
            while (j < len(s)):
                ret += s[j] # js are the backbone columns, 
                            # for n = 5, j are 0, 8, 16, ...
                if d1 != 0 and d1 != inc and (j + d1) < len(s):
                    ret += s[j+d1]
                j += inc    
                
        return ret