LeetCode "ZigZag Conversion"

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;
    }
};