38. Count and Say

38. Count and Say

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 1:

Input: 1
Output: "1"

Example 2:

Input: 4
Output: "1211"

My Solutions:

每一层都要用一次string compression

class Solution {
    public String countAndSay(int n) {
        String res = "1";
        if (n == 1) return res;
        for (int i = 1; i < n; i++) {
            res = helper(res);
        }
        return res;
    }
    
    public String helper(String s) {
        char[] arr = s.toCharArray();
        int count = 1;
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            if (i == s.length() - 1 || arr[i] != arr[i + 1]) { //has to check i == s.length() - 1 first to make sure no index out of range
                sb.append(count);
                sb.append(arr[i]);
                count = 1;
            } else {
                count++;
            }
        }
        return sb.toString();
    } 
}