ZigZag Conversion [LeetCode]

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".

Summary: just finds the index in string s of every rows.

     string convert(string s, int nRows) {
         string output;
         if(nRows <= )
             return output;
         if(nRows ==  || nRows == s.size())
             return s;

         for(int i = ; i < nRows; i ++) {
             if(i ==  || i +  == nRows){
                 int j = i;
                 while(j < s.size()){
                     output += s[j];
                     j += nRows + nRows - ;
                 }
             }else{
                 int j =  + i;
                 while(j < s.size()){
                     output += s[j];
                     int next_idx = j + nRows - (i + ) + nRows - (i + );
                     if(next_idx < s.size())
                         output += s[next_idx];
                     j += nRows + nRows - ;
                 }
             }
         }
         return output;
     }