[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 n^th sequence.

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

class Solution {
public:
    string countAndSay(int n) {
        string result("1");
        string pre(result);
        for(int i=1;i<n;i++){
            int len = pre.size();
            result.clear();
            for(int j=0;j<len;){
                char cur = pre[j];
                int start = j;
                j++;
                while(j<len && cur == pre[j])
                    j++;
                int end = j;
                int num = end - start;
                result += intToS(num);
                result.push_back(pre[start]);

            
            }//end for
           pre = result;
        }//end for
        return result;
    }//end func
private:
    string intToS(int n){
        ostringstream os;
        os<<n;
        string s;
        s = os.str();
        return s;
    }
};