zigzag conversion leetcode c++

for this problem,

you must divided the problem into two part.

then find the change rule. the precise rule please see it in the code bellow.

class Solution {
public:
    string convert(string s, int nRows) {
    if(s == "" || s.length() < nRows || nRows <=1)
        return s;
    string q = "";
    for(int i = 0;i<s.length();i += (nRows-1)*2)
        q = q + s[i];
    for(int i = 1;i<nRows-1; i++)
        for(int j = i;j<s.length();j = j+(nRows-1)*2)
        {
            q = q+s[j];
            if(j+(nRows-i-1)*2 < s.length())
                q = q+s[j+(nRows-i-1)*2];
        }
    for(int i = nRows-1;i<s.length();i += (nRows-1)*2)
        q = q + s[i];
    return q;
    }
};