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;
}
};