字符串---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.

Have you met this question in a real interview? Yes
Example
Given n = 5, return “111221”.

public class Solution {
    public String countAndSay(int n) {
        String oldString = "1";
        while (--n > 0) {
            StringBuffer s = new StringBuffer();
            char [] oldChars = oldString.toCharArray();

            for (int i = 0; i < oldChars.length; i++) {
                int count = 1;
                while ((i+1) < oldChars.length && oldChars[i] == oldChars[i+1]) {
                    count++;
                    i++;
                }
                s.append(String.valueOf(count) + String.valueOf(oldChars[i]));
            }
            oldString = s.toString();
        }

        return oldString;
    }
}