DouKaku?

using System;
using System.Collections.Generic;
static class Program {
    static void Main() {
        string[] pathList = {"aaa/bbb", "aaa/ccc", "aaa/ddd/eee", "bbb/ddd/eee", "bbb/ddd/fff"};
        Console.WriteLine("[{0}]", string.Join(", ", Ls(pathList, "aaa/")));
        Console.WriteLine("[{0}]", string.Join(", ", Ls(pathList, "aaa/ddd/")));
        Console.WriteLine("[{0}]", string.Join(", ", Ls(pathList, "bbb/")));
        Console.WriteLine("[{0}]", string.Join(", ", Ls(pathList, "bbb/ddd/")));
    }
    static string[] Ls(string[] paths, string pattern) {
        SortedList<string, string> list = new SortedList<string, string>();
        Array.ForEach(paths, delegate(string x) {
            if (x.StartsWith(pattern)) {
                x = x.Substring(pattern.Length);
                int i = x.IndexOf('/');
                list[i < 0 ? x : x.Substring(0, i + 1)] = null;
            }
        });
        return new List<string>(list.Keys).ToArray();
    }
}