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".

 1 class Solution {

 2 public:

 3     string convert(string s, int nRows) {

 4         if (s.empty() || nRows <= 1) return s;

 5         stringstream ss;

 6         int n = s.length();

 7         for (int row = 0; row < nRows; ++row) {

 8             for (int i = row; i < n; ) {

 9                 if (nRows - row - 1 > 0) {

10                     ss << s[i];

11                     i += 2 * (nRows - row - 1);

12                     if (i >= n) break;

13                     

14                 }

15                 if (row > 0) {

16                     ss << s[i];

17                     i += 2 * row;

18                     if (i >= n) break;

19                 }

20             }

21         }

22         return ss.str();

23     }

24 };