6. Z字形变换

输入: s = "PAYPALISHIRING", numRows = 3
输出: "PAHNAPLSIIGYIR"

输入: s = "PAYPALISHIRING", numRows = 4
输出: "PINALSIGYAHRPI"
解释:

P     I    N
A   L S  I G
Y A   H R
P     I

class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1) {
            return s;
        }

        List<StringBuilder> rows = new ArrayList<>();
        for (int i = 0; i < Math.min(numRows, s.length()); i++) {
            rows.add(new StringBuilder());
        }

        int curRow = 0;
        boolean goingDown = false;

        for (char c : s.toCharArray()) {
            rows.get(curRow).append(c);
            if (curRow == 0 || curRow == numRows - 1) {
                goingDown = !goingDown;
            }
            curRow += goingDown ? 1 : -1;
        }

        StringBuilder ret = new StringBuilder();
        for (StringBuilder row : rows) {
            ret.append(row);
        }
        return ret.toString();
    }
}