The appearance of the number of columns

"Appearance number of columns" is a sequence of integers, beginning with the number 1, is the description of the preceding each item in the sequence. The first five are as follows:

1.1
2.11
3.21
4.1211
5.111221
1 is read as "one 1" ( "a a"), i.e., 11.
11 is read as "two 1s" ( "two one"), i.e., 21.
21 is read as "one 2", "one 1 " ( " a two", "a a"), i.e., 1211.

Given a positive integer n (1 ≤ n ≤ 30), the n-th series term output appearance.

Note: Each sequence of integers will be represented as a string.

 

Example 1:

Input: 1
Output: "1"
Explanation: This is a basic sample.
Example 2:

Input: 4
Output: "1211"
Explanation: When n = 3, the sequence is "21", which we have "2" and "1" groups, "2" can be read as "12", i.e. occurrence frequency = = 1 and the value 2; similarly "1" can be read as "11." So the answer is "12" and "11" are combined, that is, "1211."

 1 public class T38 {
 2     public String countAndSay(int n) {
 3         String str = "1";
 4         for (int i = 2; i <= n; i++) {
 5             int count = 1;
 6             StringBuilder sb = new StringBuilder();
 7             char pre = str.charAt(0);
 8             for (int j = 1; j < str.length(); j++) {
 9                 char c = str.charAt(j);
10                 if (pre == c) {
11                     count++;
12                 } else {
13                     sb.append(count).append(pre);
14                     pre = c;
15                     count = 1;
16                 }
17             }
18             sb.append(count).append(pre);
19             str = sb.toString();
20         }
21         return str;
22 
23     }
24 }