[leetcode] Count and Say @ Python

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.

Code:

class Solution:
    # @return a string
    def countAndSay(self, n):
        #思路：遍历，每次对前面的string进行分析，输出
        s= '1'
        for i in range(n-1):
            prev = newS =''
            num = 0
            for curr in s:
                if prev !='' and prev != curr:
                    newS += str(num) + prev  # how many times the value "prev" appears
                    num = 1
                else:
                    num += 1
                prev = curr
            newS += str(num) + prev
            s = newS
        return s