Permutation 演算法（不確定）

Perm( list, sub_start, string_length)
  if (sub_start == string_length)
     print out the list
  else
   int i
    i = sub_start until string_length <- present as k
      swap(list[k], list[i]) // shift
      Perm(list, i+1, string_length)
      swap(list[k], list[i]) // recover


參考資料：http://www.experts-exchange.com/Programming/Languages/C_Sharp/Q_27308602.html

Lexicographic Permutation

額外資料：http://www.dreamincode.net/code/snippet4206.htm
http://www.freewebs.com/permute/lexico.html