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 nRows) {
        if(nRows<=1 || s.length()<=0) return s;
        StringBuilder sb = new StringBuilder();
        for(int i=0;i<nRows;i++){
            for(int j=0,index=i;index<s.length();j++,index=(nRows+nRows-2)*j+i){
                sb.append(s.charAt(index));//add red char
                if(i==0 ||i ==nRows-1)// first and last line don't have green char
                    continue;
                if(index+(nRows-i-1)*2<s.length()){
                    sb.append(s.charAt(index+(nRows-i-1)*2));
                }
            }
        }
        return sb.toString();
    }
}

 if(index+(nRows-i-1)*2<len)

index<len