Leetcode 6: ZigZag Conversion

https://leetcode.com/problems/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".

 

Code

class Solution(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        if s is None or len(s) <= numRows or numRows==1:
            return s
        
        res = ""
        for i in xrange(numRows):
            if i == 0 or i == (numRows-1):
                res += s[i::(2*numRows-2)]
            else:
                s1 = s[i::(2*numRows-2)]
                s2 = s[(2*numRows-2-i)::(2*numRows-2)]
                for c1, c2 in zip(s1, s2):
                    res += c1 + c2
                if len(s1) > len(s2):  # s1 is at most longer than s2 by one character
                    res += s1[-1]
        return res
                    
                    

 

Idea

Every `2*numRows-2` characters in `s` will form one zigzag. You can find the rules when outputing the zigzag horizontally: