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"

.

public String convert(String s, int nRows) {
	    char[] c = s.toCharArray();
	    int len = c.length;
	    StringBuffer[] sb = new StringBuffer[nRows];
	    for (int i = 0; i < sb.length; i++) sb[i] = new StringBuffer();

	    int i = 0;
	    while (i < len) {
	        for (int idx = 0; idx < nRows && i < len; idx++) // vertically down
	            sb[idx].append(c[i++]);
	        for (int idx = nRows-2; idx >= 1 && i < len; idx--) // obliquely up
	            sb[idx].append(c[i++]);
	    }
	    for (int idx = 1; idx < sb.length; idx++)
	        sb[0].append(sb[idx]);
	    return sb[0].toString();
	}