對List內元素子集合進行組合排列

對List中元素子集合進行組合排列，使用了遞歸，代碼如下：

using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Collections;   namespace ConsoleApplication2 {     class Program     {         static void Main(string[] args)         {                                     CombinationsPermutations("asdf", 3);                 }           private static void RotateLeft<T>(IList<T> sequence, int start, int count)         {             T tmp = sequence[start];             sequence.RemoveAt(start);             sequence.Insert(start + count - 1, tmp);         }           public static IEnumerable<IList<T>> Combinations<T>(             IList<T> sequence,             int start,             int count,             int choose)         {             if (choose == 0) yield return sequence;             else             {                 for (int i = 0; i < count; i++)                 {                     foreach (var perm in Combinations(                                                 sequence,                                                 start + 1,                                                 count - 1 - i,                                                 choose - 1))                         yield return perm;                     RotateLeft(sequence, start, count);                 }             }