38. Count and Say

StringBuilder (my solution)

class Solution {
    public String countAndSay(int n) {
        String result = "1";
        while (n > 1) {
            result = helper(result);
            n--;
        }
        return result;
    }

    private String helper(String s) {
        int n = s.length();
        int pos = 0;
        StringBuilder sb = new StringBuilder();
        while (pos < n) { 
            char c = s.charAt(pos);
            int cnt = 1;
            while ((pos + 1 < n) && s.charAt(pos + 1) == c) {
                cnt++;
                pos++;
            }
            sb.append(String.valueOf(cnt));
            sb.append(c);
            pos++;
        }
        return sb.toString();
    }
}

Remarks:

1. TC: $O(2^n)$
2. Don’t forget to pos++ at the end of the while loop
3. In the while statement, first check pos + 1 < n then check s.charAt(pos + 1).
   Java handles logic expression LEFT to RIGHT.