6. ZigZag Conversion

# 12:30 -- 13:31:
# run time: 102ms, 76%; 112ms, 64%; 95ms, 87%
class Solution(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        length = len(s)
        if numRows <= 1 or length <= numRows:
            return s
        numset = 2 * numRows -2
        temp = 0
        output = ''
        while temp <= numRows - 1:
            if temp == 0 or temp == numRows- 1:
                start = temp
                while start < len(s):
                    output += s[start]
                    start += numset
            else:
                start = temp
                end = numset - start
                while start < ( length - 1) / numset * numset:
                    output += s[start]
                    output += s[end]
                    start += numset
                    end += numset
                if length > start:
                    output += s[start]
                if length > end:
                    output += s[end]
            temp += 1
        return output