Leetcode#6 ZigZag Conversion

class Solution {
    public:
        string convert(string s, int numRows) {
            if (numRows == 1)
                return s;

            vector<vector<char>> res(numRows, vector<char>(0, 0));

            int factor = numRows + numRows - 2;
            for(int i=0; i<s.size(); i++) {
                int index = i%factor;
                int row_idx = index < numRows ? index : (factor-index);
                res[row_idx].push_back(s[i]);
            }

            string tmp = "";
            for (int i = 0; i < numRows; i++){
                for(int j=0; j < res[i].size();j++){
                    tmp = tmp + res[i][j];
                }
            }
            return tmp;
        }
};