Concatenation of Zig-Zag String in n Rows

class Solution{

    public:

    string convert(string s, int n) {      

        if(n<=1) return s;

        vector<string> res(n,"");

        int p=0;

        bool flag = true;

        for(int i=0; i<s.size(); i++)

        {

            res[p].push_back(s[i]);

            if(flag) p++;

            else p--;

            if(p==0) flag = true;

            if(p==n-1) flag = false;

        }

        string ans ="";

        for(string it : res) 

        {

            ans += it;

        }

        return(ans);

    }

};