LeetCode 6 - 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".

// P   I   N
// A  LS  IG
// Y A H R
// P   I
public String convert(String s, int nRows) {
    if(nRows <=1) return s;
    StringBuilder sb = new StringBuilder();
    int len = s.length();
    int step = (nRows-1) * 2;
    for(int i=0; i<nRows; i++) {
        int j = i;
        while(j < len) {
            sb.append(s.charAt(j));
            if(!(i==0 || i==nRows-1)) {
                int k = j+ 2*(nRows-i-1);
                if(k<len)
                    sb.append(s.charAt(k));
            }
            j += step;
        }
    }
    return sb.toString();
}