[Leetcode]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".

把一個字符串按Z字行排列，然後按每行順序輸出所有字母~這種題主要是找出規律來~

class Solution:
    # @return a string
    def convert(self, s, nRows):
        if s is None or len(s) == 0 or nRows <= 0: return ""
        if nRows == 1: return s
        step = 2 * nRows - 2; res = ""
        for i in xrange(nRows):
            for j in xrange(i, len(s), step):
                res += s[j]
                if i > 0 and i < nRows - 1 and j + step - 2 * i < len(s):
                    res += s[j + step - 2 * i]
        return res