C#查找字符串所有排列组合的方法

// 1. remove first char 
// 2. find permutations of the rest of chars
// 3. Attach the first char to each of those permutations.
// 3.1 for each permutation, move firstChar in all indexes 
//   to produce even more permutations.
// 4. Return list of possible permutations.
public string[] FindPermutations(string word)
{
 if (word.Length == 2)
 {
  char[] _c = word.ToCharArray();
  string s = new string(new char[] { _c[1], _c[0] });
  return new string[]
  {
   word,
   s
  };
 }
 List<string> _result = new List<string>();
 string[] _subsetPermutations = FindPermutations(word.Substring(1));
 char _firstChar = word[0];
 foreach (string s in _subsetPermutations)
 {
  string _temp = _firstChar.ToString() + s;
  _result.Add(_temp);
  char[] _chars = _temp.ToCharArray();
  for (int i = 0; i < _temp.Length - 1; i++)
  {
   char t = _chars[i];
   _chars[i] = _chars[i + 1];
   _chars[i + 1] = t;
   string s2 = new string(_chars);
   _result.Add(s2);
  }
 }
 return _result.ToArray();
}