下面是一个coffeescript实现

combinations: (list, n) ->
        permuations = Math.pow(2, list.length) - 1
        out = []
        combinations = []

        while permuations
            out = []

            for i in [0..list.length]
                y = ( 1 << i )
                if( y & permuations and (y isnt permuations))
                    out.push(list[i])

            if out.length <= n and out.length > 0
                combinations.push(out)

            permuations--

        return combinations 

也许我错过了重点(你需要的是算法，而不是现成的解决方案)，但看起来scala已经开箱即用了(现在):

def combis(str:String, k:Int):Array[String] = {
  str.combinations(k).toArray 
}

使用这样的方法:

  println(combis("abcd",2).toList)

会产生:

  List(ab, ac, ad, bc, bd, cd)

我已经编写了一个类来处理处理二项式系数的常见函数，这是您的问题属于的问题类型。它执行以下任务:

Outputs all the K-indexes in a nice format for any N choose K to a file. The K-indexes can be substituted with more descriptive strings or letters. This method makes solving this type of problem quite trivial. Converts the K-indexes to the proper index of an entry in the sorted binomial coefficient table. This technique is much faster than older published techniques that rely on iteration. It does this by using a mathematical property inherent in Pascal's Triangle. My paper talks about this. I believe I am the first to discover and publish this technique, but I could be wrong. Converts the index in a sorted binomial coefficient table to the corresponding K-indexes. Uses Mark Dominus method to calculate the binomial coefficient, which is much less likely to overflow and works with larger numbers. The class is written in .NET C# and provides a way to manage the objects related to the problem (if any) by using a generic list. The constructor of this class takes a bool value called InitTable that when true will create a generic list to hold the objects to be managed. If this value is false, then it will not create the table. The table does not need to be created in order to perform the 4 above methods. Accessor methods are provided to access the table. There is an associated test class which shows how to use the class and its methods. It has been extensively tested with 2 cases and there are no known bugs.

要了解这个类并下载代码，请参见将二项式系数表化。

将这个类转换为c++应该不难。

Lisp宏为所有值r(每次取)生成代码

(defmacro txaat (some-list taken-at-a-time)
  (let* ((vars (reverse (truncate-list '(a b c d e f g h i j) taken-at-a-time))))
    `(
      ,@(loop for i below taken-at-a-time 
           for j in vars 
           with nested = nil 
           finally (return nested) 
           do
             (setf 
              nested 
              `(loop for ,j from
                    ,(if (< i (1- (length vars)))
                         `(1+ ,(nth (1+ i) vars))
                         0)
                  below (- (length ,some-list) ,i)
                    ,@(if (equal i 0) 
                          `(collect 
                               (list
                                ,@(loop for k from (1- taken-at-a-time) downto 0
                                     append `((nth ,(nth k vars) ,some-list)))))
                          `(append ,nested))))))))

So,

CL-USER> (macroexpand-1 '(txaat '(a b c d) 1))
(LOOP FOR A FROM 0 TO (- (LENGTH '(A B C D)) 1)
    COLLECT (LIST (NTH A '(A B C D))))
T
CL-USER> (macroexpand-1 '(txaat '(a b c d) 2))
(LOOP FOR A FROM 0 TO (- (LENGTH '(A B C D)) 2)
      APPEND (LOOP FOR B FROM (1+ A) TO (- (LENGTH '(A B C D)) 1)
                   COLLECT (LIST (NTH A '(A B C D)) (NTH B '(A B C D)))))
T
CL-USER> (macroexpand-1 '(txaat '(a b c d) 3))
(LOOP FOR A FROM 0 TO (- (LENGTH '(A B C D)) 3)
      APPEND (LOOP FOR B FROM (1+ A) TO (- (LENGTH '(A B C D)) 2)
                   APPEND (LOOP FOR C FROM (1+ B) TO (- (LENGTH '(A B C D)) 1)
                                COLLECT (LIST (NTH A '(A B C D))
                                              (NTH B '(A B C D))
                                              (NTH C '(A B C D))))))
T

CL-USER> 

And,

CL-USER> (txaat '(a b c d) 1)
((A) (B) (C) (D))
CL-USER> (txaat '(a b c d) 2)
((A B) (A C) (A D) (B C) (B D) (C D))
CL-USER> (txaat '(a b c d) 3)
((A B C) (A B D) (A C D) (B C D))
CL-USER> (txaat '(a b c d) 4)
((A B C D))
CL-USER> (txaat '(a b c d) 5)
NIL
CL-USER> (txaat '(a b c d) 0)
NIL
CL-USER> 

这是我对javascript的贡献(没有递归)

set = ["q0", "q1", "q2", "q3"]
collector = []


function comb(num) {
  results = []
  one_comb = []
  for (i = set.length - 1; i >= 0; --i) {
    tmp = Math.pow(2, i)
    quotient = parseInt(num / tmp)
    results.push(quotient)
    num = num % tmp
  }
  k = 0
  for (i = 0; i < results.length; ++i)
    if (results[i]) {
      ++k
      one_comb.push(set[i])
    }
  if (collector[k] == undefined)
    collector[k] = []
  collector[k].push(one_comb)
}


sum = 0
for (i = 0; i < set.length; ++i)
  sum += Math.pow(2, i)
 for (ii = sum; ii > 0; --ii)
  comb(ii)
 cnt = 0
for (i = 1; i < collector.length; ++i) {
  n = 0
  for (j = 0; j < collector[i].length; ++j)
    document.write(++cnt, " - " + (++n) + " - ", collector[i][j], "<br>")
  document.write("<hr>")
}   

static IEnumerable<string> Combinations(List<string> characters, int length)
{
    for (int i = 0; i < characters.Count; i++)
    {
        // only want 1 character, just return this one
        if (length == 1)
            yield return characters[i];

        // want more than one character, return this one plus all combinations one shorter
        // only use characters after the current one for the rest of the combinations
        else
            foreach (string next in Combinations(characters.GetRange(i + 1, characters.Count - (i + 1)), length - 1))
                yield return characters[i] + next;
    }
}