leetcode 38. Count and Say

class Solution {
public:
    string countAndSay(int n) {
        if(n < 0) return "";
        string ans = "1";
        string res = "";
        for(int i = 1;i < n; i++){
            int count = 0,j = 0,l=0 ;
            res ="";
            while(l < ans.size()){
                while(ans[j] == ans[l] && j < ans.size()){
                    count++;
                    j++;
                }
                int f = 0;
                while(count > 0){
                    f = f*10+count % 10;
                    count/=10;
                }
                while(f > 0){
                    res += char('0'+f%10);
                    f /= 10;
                }
                res += ans[l];
                l = j;
               
            }
            swap(res,ans);
        }
        return ans;
    }
};