38. Count and Say

Posted 2021-01-09 bernieloveslife

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 where 1 ≤ n ≤ 30, 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"

class Solution:
    def countAndSay(self, n):
        """
        :type n: int
        :rtype: str
        """
        def say(s):
            res = ‘‘
            i = 0
            while i<len(s):
                temp = s[i]
                count = 1
                while i<len(s)-1 and s[i]==s[i+1]: #这里刚开始写的是while s[i]==s[i+1] and i<len(s)-1: ,总会报错数组越界。反过来就可以了，因为判断前一个为否之后python就不会去看后边的这个式子了
                    count += 1
                    i += 1
                res = res + str(count) + temp
                i += 1
            return res
        s = ‘1‘
        if n==1:
            return s
        while n>1:
            s = say(s)
            n -= 1
        return s