Leetcode#6 ZigZag Conversion

 1 string convert(string s, int nRows) {
 2         string res;
 3         
 4         if (nRows <= 1)
 5             return s;
 6         
 7         for (int r = 0; r < nRows; r++) {
 8             int curr = r;
 9             int prev = -1;
10             bool toggle = true;
11             while (curr < s.length()) {
12                 if (prev != curr)
13                     res += s[curr];
14                 prev = curr;
15                 curr += toggle ? 2 * (nRows - r - 1) : 2 * r;
16                 toggle = !toggle;
17             }
18         }
19         
20         return res;
21 }