leetcode 6. ZigZag Conversion

It's too tired preparing for interview while working. So I had a totally relaxing, leetcode-free weekend.
As for leetcode 6, the most stable way to solve this problem is attached below. And I didn't try to figure out the formula with a lot of % like when I was at my 20's.

class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1) return s;
        StringBuilder[] ret = new StringBuilder[numRows];
        for (int i = 0; i < numRows; ++i) ret[i] = new StringBuilder();
        int row = 0;
        int dir = 1;
        for (int i = 0; i < s.length(); ++i) {
            ret[row].append(s.charAt(i));
            row += dir;
            if (row == numRows - 1) dir = -1;
            else if (row == 0) dir = 1;
        }
        String result = "";
        for (StringBuilder sb: ret) {
            result += sb.toString();
        }
        return result;
    }
}