Leetcode: 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.

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

Use a string to store the previous result for every calculation. Because the string could be very long, it's more efficient to convert the sting to a char array, instead of simply use String.charAt(i) method. Proof can be found here Fastest way to iterate over all the chars in a String

public class Solution {
    public String countAndSay(int n) {
        String old = "1";
        for (int i = 1; i < n; i++) {
            StringBuilder sb = new StringBuilder();
            char[] chars = old.toCharArray();
            for (int j = 0; j < old.length(); j++) {
            	int times = 1;
                while (j < old.length() - 1 && chars[j] == chars[j + 1]) {
                	times++;
                	j++;
                }
                sb.append(times).append(chars[j]);
            }
            old = sb.toString();
        }
        return old;
    }
}