class Solution {public:    string convert(string s, int nRows) {        if(s.empty() || nRows == 1) return s;        vector<string> rec;        int cnt = nRows;        while (cnt--)    rec.push_back("");        int inx = 0, step = 1;        for (int i = 0; i < s.length(); i++)        {            rec[inx] += s[i];            inx += step;            if (inx == nRows || inx == -1)            {                                step = -step;                inx += step * 2;            }        }        //        string r;        for (int i = 0; i < nRows; i++) r += rec[i];        return r;    }};