leetcode 6. ZigZag Conversion

给一串N型排列，把它转换成行排列。直接模拟，可以注意到，第一行和最后一行，间距都是2*step-2,第二行开始两个间距为2*step-4,2然后第一个间距依次＋2，第二个间距依次－2

class Solution(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        if (numRows == 1) : return s

        res = ''
        lenS = len(s)
        step = numRows * 2 - 2
        res = s[::step]

        for i in range(1,numRows - 1):
            for j in range(i,lenS,step):
                res = res + s[j]
                if j + (step - i * 2) < lenS : 
                    res = res + s[j + step - i * 2]
            
        i = numRows -1 
        res = res + s[i::step]
        return res