38. Count and Say

class Solution(object):
    def countAndSay(self, n):
        """
        :type n: int
        :rtype: str
        """
        res = '1'
        while n > 1:
            n -= 1
            pre = res
            res = ''
            cnt = 1
            s = pre[0]
            for i in range(1, len(pre)):
                if pre[i] == s:
                    cnt += 1
                else:
                    res += str(cnt) + s
                    s = pre[i]
                    cnt = 1
            res += str(cnt) + s
        return res