zigzag Convert

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) {
int len = s.length();
if (len == 0 || nRows <= 1) return s;

String[] ans = new String[nRows];
Arrays.fill(ans, "");
int row = 0, delta = 1;
for (int i = 0; i < len; i++) {
ans[row] += s.charAt(i);
row += delta;
if (row >= nRows) {
row = nRows-2;
delta = -1;
}
if (row < 0) {
row = 1;
delta = 1;
}
}

String ret = "";
for (int i = 0; i < nRows; i++) {
ret += ans[i];
}
return ret;
}