830. 较大分组的位置

class Solution {
   public List> largeGroupPositions(String S) {
        List> ls = new LinkedList>();
        for (int i = 0; i < S.length() - 2; i++) {
            if (S.charAt(i) == S.charAt(i + 1) && S.charAt(i) == S.charAt(i + 2)) {
                char s = S.charAt(i);
                List l = new LinkedList();
                int a = i;
                int b = i;
                while (b<S.length()&&s == S.charAt(b)) {
                    b++;
                }
                l.add(a);
                l.add(b-1);
                ls.add(l);
                i = b-1;
            }
        }
        return ls;
    }
}