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(nRows == 1)
            return s;
        string res;
        int step = (nRows - 1) * 2;
        int N = s.size();
        
        for(int i = 0; i < nRows; i++) {
            int j = 0;
            while(1) {
                if(i > 0 && i < nRows-1 && j-i >= 0 && j-i < N)
                    res.push_back(s[j-i]);
                if(i+j < N) 
                    res.push_back(s[j+i]);
                if(i+j >= N) break;
                j += step;
            }
        }
        return res;
    }
};