class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1) return s;
       
        List<StringBuilder> rows = new ArrayList<>();
       
        int currentRow = 0;
        boolean direction = false;
        for (char c : s.toCharArray()) {
            if (rows.size() == currentRow) {
                rows.add(new StringBuilder());
            }
            rows.get(currentRow).append(c);
           
            if (currentRow == 0 || currentRow == numRows - 1) {
                direction = !direction;
            }
           
            currentRow += direction ? 1 : -1;
        }
       
        StringBuilder result = new StringBuilder();
       
        for (StringBuilder row : rows) {
            result.append(row);
        }
        return result.toString();
    }
}