[LeetCode] ZigZag Conversion

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

P   A   H   N
A P L S I I G
Y   I   R

And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string text, int nRows);

convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".

class Solution {
public:
    string convert(string s, int nRows) {
        string s0;
        vector<string> vs(nRows,s0);
        int len = s.size();
        for(int i=0;i<len;){
            for(int j=0;j<nRows && i<len;j++){
                vs[j].push_back(s[i]);
                i++;
            }
            for(int k=nRows-2;k>0 && i<len;k--){
                vs[k].push_back(s[i]);
                i++;
            }
        }
        string result;
        for(int i=0;i<nRows;i++){
           result += vs[i];
        }
        return result;
    }//end func
};