[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".

Pasted from <http://leetcode.com/onlinejudge#question_1>

Analysis:

1P		9H
2A	8S	10I	16
3Y	7I	11N	15
4P	6L	12G	14
5A		13

j = nRows*2 -2 if i = 0 || nRows-1, 

j = (nRows-i)*2  if i>0 && i<nRows-1, c is odd

j = i*2 c is even

class Solution {
public:
    string convert(string s, int nRows) {
        
        if(nRows <= 1) return s;
        
        string str;
        for(int i=0; i < nRows; i++)
        {
            int j = i;    
            int c = 0;
            while( j < s.size())
            {
                str.push_back(s[j]);
                c++;
                if (i == 0 || i == nRows - 1)
                    j = j + nRows + nRows - 2;
                else 
                {
                    if (c % 2 == 1)
                        j = j + (nRows-i - 1) * 2;
                    else
                        j = j + i * 2;
                }
            }
        }
        
        return str;
    }
};