[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) 
    {
        if(1 == nRows) return s;
        string res;
        int len = s.length();
        for(int i=0;i<nRows;i++)
        {
            for(int k = i; k<len; k+=(2*nRows - 2))
            {
                res += s[k];
                int v = k + 2*(nRows - 1 - i);
                if(i != 0 && i != nRows - 1 && v < len)
                    res += s[v];
            }
        }
        return res;
    }
};