38. Count and Say leetcode python 2016 new season

The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 111221, ...

1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n, generate the nth sequence.

Note: The sequence of integers will be represented as a string.

class Solution(object):
    def countAndSay(self, n):
        """
        :type n: int
        :rtype: str
        """
        if n == 1:
            return '1'
        else:
            prev = self.countAndSay(n - 1)
            start = 0
            res = ''
            while start < len(prev):
                count = 0
                cur = prev[start]
                while start < len(prev) and cur == prev[start]:
                    start += 1
                    count += 1
                res += str(count) + cur
            return res