给定一个集合，求出其所有子集合

code demonstrate using c#
using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        /**//// <summary>
        /// 函数：给定一个集合，求出其所有子集合
        /// </summary>
          private static List<string> printList(string[] arr, int num)
        {
            if (num < 0)
            {
                List<string> reto = new List<string>();
                reto.Add("");
                return reto;
            }
            else
            {
                List<string> ret = printList(arr, num - 1);
                List<string> addRet = new List<string>();
                ret.ForEach(delegate(string x) { addRet.Add(x +" "+ arr[num]); });
                ret.AddRange(addRet);
                return ret;
            }
        }
        static void Main(string[] args)
        {
            //演示
            string[] arr ={ "a", "b", "c"};
            List<string> ret = printList(arr, 2);
            ret.ForEach(delegate(string x) { Console.WriteLine(x); });
            Console.Read();
        }
    }
}