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".

Analysis: 

public class Solution {
    public String convert(String s, int nRows) {
        if(nRows==1) return s;
        
        StringBuilder res = new StringBuilder();
        int acc = 2*nRows-2;    // nRows==1 -> acc=0, infinite loop
        int[] diff = new int[nRows];
        for(int i=0; i<nRows; i++) {
            if(i==0 || i==nRows-1) diff[i]=acc;
            else diff[i]=acc-i*2;
        }
        
        for(int j=0; j<nRows; j++) {
            int first=diff[j], second=diff[diff.length-1-j], k=j;
            boolean odd = true;
            while(k < s.length()) {
                res.append(s.charAt(k));
                if(odd) {
                    k += first;
                    odd = false;
                }
                else {
                    k += second;
                    odd = true;
                }
            }
        }
        
        return res.toString();
    }
}