两两组合算法-递归实现方法

count = len(len-1)/n
arr={1,2,3}
len=3
n=2
组合的个数 count = len(len-1)/2 = 3(3-1)/2 =3
---------------------- 
得到的组合结果为
1,2
1,3
2,3
----------------------
using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;namespace Testing
 {
   class Program
   {
     static void Main(string[] args)
     {
       TestDoubleComposition();      Console.ReadLine();
     }    static List<string> m_list;
     /// <summary>
     /// 两两组合算法
     /// </summary>
     static void TestDoubleComposition()
     {
       m_list = new List<string>();
       List<string> list = new List<string>();
       list.Add("1");
       list.Add("2");
       list.Add("3");
       DoubleComposition(list);
       m_list.ForEach(o => {
         Console.WriteLine(o);
       });
     }    /// <summary>
     /// 两两组合算法
     /// </summary>
     static void DoubleComposition(List<string> arr)
     {
       if (arr == null || arr.Count <= 1)
         return;      string str = arr.First();
       arr.RemoveAt(0);
       arr.ForEach(o =>
       {
         m_list.Add(string.Format("{0},{1}",str,o));
       });      DoubleComposition(arr);
     }
   }
 }