Z字形变换

 1     public String convert(String s, int numRows) {
 2         if (s == null || numRows == 1) {
 3             return s;
 4         }
 5         
 6         //每行一个StringBuilder对象
 7         List<StringBuilder> list = new ArrayList<>();
 8         for (int i = 0; i < Math.min(s.length(), numRows); i++) {
 9             list.add(new StringBuilder());
10         }
11 
12         boolean down = false;
13         int row = 0;
14         //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
15         for (int i = 0; i < s.length(); i++) {
16             if (row == 0 || row == list.size() - 1) {
17                 down = !down;
18             }
19             list.get(row).append(s.charAt(i));
20             row += (down ? 1 : -1);
21         }
22         String res = "";
23         for (StringBuilder sb : list) {
24             res += sb.toString();
25         }
26         return res;
27     }