6. Z 字形变换

6. Z 字形变换

class Solution {
    public String convert(String s, int numRows) {
        if (s == null || numRows <= 0 || s.equals(""))
            return "";
        if (numRows == 1)
            return s;
        List<StringBuilder> rows = new ArrayList<>();
        for (int index = 0; index < numRows; index++)
            rows.add(new StringBuilder());
        int flag = -1;
        int i = 0;
        for (char ch : s.toCharArray()) {
            rows.get(i).append(ch);
            if (i == 0 || i == numRows - 1)
                flag = -flag;
            i += flag;
        }
        StringBuilder ans = new StringBuilder();
        for (StringBuilder currRow : rows)
            ans.append(currRow);
        return ans.toString();
    }
}