6. ZigZag Conversion

class Solution {
    public String convert(String s, int numRows) {
        if (s == null || numRows == 1 || s.length() <= numRows) {
            return s;
        }

        StringBuilder result = new StringBuilder();

        for (int row = 0; row < numRows; row++) {
            int step1 = (numRows - row - 1) * 2;
            int step2 = row * 2;
            int pos = row;
            result.append(s.charAt(pos));

            while (true) {
                pos += step1;
                if (pos >= s.length()) {
                    break;
                }
                if (step1 > 0) {
                    result.append(s.charAt(pos));
                }
                pos += step2;
                if (pos >= s.length()) {
                    break;
                }
                if (step2 > 0) {
                    result.append(s.charAt(pos));
                }
            }
        }

        return result.toString();
    }
}