Find the Sequence of Strings Appeared on the Screen

class Solution { public List<String> stringSequence(String target) { List<String> list = new ArrayList<>(); StringBuilder sb = new StringBuilder(); for(char c : target.toCharArray()){ if(list.size()!=0){ sb.append(list.get(list.size()-1)); } sb.append('a'); list.add(sb.toString()); for(char i = 'a'+1;i<=c;i++){ sb.deleteCharAt(sb.length()-1); sb.append(i); list.add(sb.toString()); } sb.setLength(0); } return list; } }