[LeetCode] 38. Count and Say_Easy

The count-and-say sequence is the sequence of integers with the first five terms as following:

1.     1
2.     11
3.     21
4.     1211
5.     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 term of the count-and-say sequence.

Note: Each term of the sequence of integers will be represented as a string.

Example 1:

Input: 1
Output: "1"

Example 2:

Input: 4
Output: "1211"

1. Constraints
1)n >= 1 edge case n == 1: return '1'

2. Ideas
recursive helper() , helper()是一个能count and say 之前的string

3. Code

class Solution(object):
    def countAndSay(self, n):
        """
        :type n: int
        :rtype: str
        """
        def helper(n, s):
            if n == 1: return s
            pre, i , temp, count = s[0], 0, "", 0
            while i < len(s):
                if s[i] == pre:
                    count += 1   
                else:
                    temp += str(count) + pre
                    count = 1
                    pre = s[i]
                i += 1
                if i == len(s):
                    temp += str(count) +pre
            return helper(n-1, temp)
        ans = ""
        return helper(n, '1')