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 class Solution {
    public String convert(String s, int numRows) {
        char[] c = s.toCharArray();
        int len = c.length;
        StringBuffer[] sb = new StringBuffer[numRows];
        for(int i =0 ;i<numRows;i++)
        {
            sb[i]=new StringBuffer();
        }
        
        int i = 0;
        while(i<len)
        {
            for(int idx=0;idx<numRows&&i<len;idx++)
            {
                sb[idx].append(c[i++]);
            }
            for(int idx=numRows-2;idx>=1&&i<len;idx--)
            {
                sb[idx].append(c[i++]);
            }
        }
        for (int idx = 1; idx < sb.length; idx++)
        sb[0].append(sb[idx]);
        return sb[0].toString();
        
    }
}