[LeetCode]Count and Say

The count-and-say sequence is the sequence of integers beginning as follows:
1, 11, 21, 1211, 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 sequence.

Note: The sequence of integers will be represented as a string.

这道题很简单啦，比如1211，它是由1个1，1个2,2个1构成的，那么它的下一个就是111221，也就是由相同连续数字的个数+数字组合而成。说到底，就是对字符串的操作。
下面贴上代码：

class Solution {
public:
    string countAndSay(int n) {
        if (n <= 0)
            return string();
        string ans="1";
        for (int i = 1; i < n; i++){
            ans = fun(ans);
        }
        return ans;
    }

    string fun(string s){
        stringstream ans;
        int count = 0;
        char c = s[0];
        for (int i = 0; i <= s.length(); i++){
            if (c!=s[i]){
                ans << count <