38. Count and Say

Leetcode String

题目

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"

题意分析：

本题是将数字从1开始，将当前数字转化为口语对应的数字。比如1口语是1个1，记作11；11读作2个1，记作21；21读作1个2，1个1，记作1211……

'1'是第一个数字，根据输入的数字n，计算第n个这样的数字。

解答

class Solution(object):
    def say(self, nn):
        """
        :type nn: str
        :rtype: str
        """

        i = 0
        result = ""

        while i < len(nn):

            count =  1       

            while i+1 < len(nn) and nn[i] == nn[i+1]:
                count += 1
                i += 1

            result += str(count)
            result += str(nn[i])

            i += 1

        return result

    def countAndSay(self, n):
        """
        :type n: int
        :rtype: str
        """

        if n == 1: return str(1)
        result = str(1)
        for i in range(1, n):
            result = self.say(result)

        return result