list<String> names = folders.map(_.getName)
SortedSet<String> set = new TreeSet<>();
set.addAll(names)

package test;

import lombok.AllArgsConstructor;
import lombok.Data;

import java.util.ArrayList;
import java.util.List;

public class Test {

    @Data
    @AllArgsConstructor
    static class State {
        int index = 0;
        List<List<String>> remaining = new ArrayList<>();
        List<String> sorted = new ArrayList<>();
    }

    public static void main(String[] args){
        List<String> folders1 = new ArrayList<String>() {{
            add("A");
            add("B");
            add("C");
        }};
        List<String> folders2 = new ArrayList<String>() {{
            add("A1");
            add("B1");
        }};
        List<String> folders3 = new ArrayList<String>() {{
            add("A2");
            add("B2");
            add("C2");
            add("D2");
        }};
        List<String> folders4 = new ArrayList<String>() {{
            add("A3");
        }};

        List<List<String>> all = new ArrayList<List<String>>() {{
            add(folders1);
            add(folders2);
            add(folders3);
            add(folders4);
        }};
        State state = new State(0, all, new ArrayList<>());
        System.out.println(sort(state));
    }

    static List<String> sort(State state) {
        if(state.remaining.size() == 0) return state.sorted;
        else {
            List<String> sorted = state.sorted;
            List<List<String>> remaining = new ArrayList<>();
            for (List<String> list : state.remaining) {
                if(list.size() > state.index) {
                    sorted.add(list.get(state.index));
                    remaining.add(list);
                }
            }
            return sort(new State(state.index + 1, remaining, sorted));
        }
    }
}