leetcode 38. Count and Say

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"

题目大意：输入一个整数n，返回第n个符合规律的数，n=1, 返回“1”， n=2,返回"11".

思路一：迭代

class Solution {
public:
    string countAndSay(int n) {
        string curr_str = "1";
        for (int i = 0; i < n - 1; i++) {
            string buffer = "";
            for (int index = 0; index < curr_str.size(); index++) {
                int cnt = 1; 
                //统计有多少个与当前所指字符相同的字符
                while (index < curr_str.size() && curr_str[index + 1] == curr_str[index]) {
                    index++;
                    cnt++;
                }
                buffer += to_string(cnt);
                buffer += curr_str[index];
            }
            curr_str = buffer;
        }
        return curr_str;
    }
};

思路二：递归

class Solution {
public:
    string countAndSay(int n) {
        if (n == 1)
            return "1";
        string res, tmp = countAndSay(n - 1);
        int len = tmp.length();
        for (int i = 0; i < len; i++) {
            int cnt = 1;
            while (i < len && tmp[i + 1] == tmp[i]) {
                cnt++;
                i++;
            }
            res += to_string(cnt);
            res += tmp[i];
        }
        return res;
    }
};