Z字形变换

    public String convert(String s, int numRows) {
        if (s == null || numRows == 1) {
            return s;
        }
        
        //每行一个StringBuilder对象
        List<StringBuilder> list = new ArrayList<>();
        for (int i = 0; i < Math.min(s.length(), numRows); i++) {
            list.add(new StringBuilder());
        }

        boolean down = false;
        int row = 0;
        //遍历字符串，一次顺序放入一行，但是在首末两行判断转向即可
        for (int i = 0; i < s.length(); i++) {
            if (row == 0 || row == list.size() - 1) {
                down = !down;
            }
            list.get(row).append(s.charAt(i));
            row += (down ? 1 : -1);
        }
        String res = "";
        for (StringBuilder sb : list) {
            res += sb.toString();
        }
        return res;
    }