Leetcode之Count and Say

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 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"

代码：

class Solution {
public:
    string countAndSay(int n) {
        if (n < 1 || n>30) return "";
	if (n == 1)return "1";
	string last = countAndSay(n - 1);
	char c='.';
	string res = "";int count = 0;
	for (int i = 0; i < last.length(); i++) {
		
		if (c != last[i]) {
			if (count != 0) {
				string s1 = to_string(count);
				s1.push_back(c);
				res += s1 ;
		   }
			c = last[i];
			count = 1;
		}
		else count++;
	}

	
	string s1 = to_string(count);
	s1.push_back(c);
	res += s1 ;
	return res;
    }
};

注：

1、使用递归的思路求解

2、学会不同类型之间的转换：int 转为 string---to_string()，char转为string---s.push_back()