Count and Say

The count-and-say sequence is the sequence of integers beginning as follows:

1, 11, 21, 1211, 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 sequence.

Example
Given n = 5, return "111221".

Note
The sequence of integers will be represented as a string.

public class Solution {
    /**
     * @param n the nth
     * @return the nth sequence
     */
    public String countAndSay(int n) {
        if (n <= 0) return null;

        String s = "1";
        for (int i = 1; i < n; i++) {
            int count = 1;
            StringBuilder sb = new StringBuilder();
            int sLen = s.length();
            for (int j = 0; j < sLen; j++) {
                if (j < sLen - 1 && s.charAt(j) == s.charAt(j + 1)) {
                    count++;
                } else {
                    sb.append(count + "" + s.charAt(j));
                    // reset
                    count = 1;
                }
            }
            s = sb.toString();
        }

        return s;
    }
}