The count-and-say sequence is the sequence of integers with the first five terms as following:

1.     1
2.     11
3.     21
4.     1211
5.     111221
1 is read off as "one 1" or 11.
11 is read off as "two 1s" or 21.
21 is read off as "one 2, then one 1" or 1211.

Given an integer n, generate the nth term of the count-and-say sequence.
Note: Each term of the sequence of integers will be represented as a string.

Example:
Input: 4
Output: "1211"

Solution：循环input=n次 按照规律建立出result，mainly by constantly counting say and append str(count) + say to the result.

Solution Code：

class Solution {
    public String countAndSay(int n) {
        StringBuilder prev = new StringBuilder("1");
        StringBuilder cur = prev;
        for(int i = 1; i < n; i++) {
            cur = new StringBuilder();

            // main process of counting
            char say = prev.charAt(0);
            int count = 0;
            for(int c = 0; c < prev.length(); c++) {
                if(prev.charAt(c) != say) {
                    cur.append(count).append(say);
                    count = 1;
                    say = prev.charAt(c);
                }
                else {
                    count ++;
                }
            }
            cur.append(count).append(say);
            prev = cur;
        }
        return cur.toString();
    }
}