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"

.

string convert(string s, int nRows) {

if(nRows <= 1) return s;
string result;
if(s.size() ==0) return result;
for(int i =0; i< nRows; i++)
{
for(int j =0, index =i; index < s.size();
j++, index = (2*nRows-2)*j +i)
{
result.append(1, s[index]);
if(i ==0 || i == nRows-1)
{
continue;
}
if(index+(nRows- i-1)*2 < s.size())
{
result.append(1, s[index+(nRows- i-1)*2]);
}
}
}
return result;
}