Storing strings in a list when some condition is true

[Heime]

I am using this function to generate permutations of characters in a string.
The algorithm uses recursion, but the final string that is needed is the one
at which the condition (= i length) holds true (currently I just call message).

Suppose one starts with (permute "word" 0 4), different strings are generated
but the ones I need saved are the ones for which (= i length) is satisfied.

How can put all the words generated in a list ?

(defun permute (string i length)
  "Generate permutations of STRING."

  (if (= i length)

      (message "%s" string)

    (let ( (j i) )
      (while (< j length)
        (swap-chars string i j)
        (permute string (1+ i) length)
        (swap-chars string i j)
        (setq j (1+ j))) )))

where swap-chars is

(defun swap-chars (string p q)
  "Swap chars in STRING at positions P and Q.
This is a destructive operation."

  (aset string p (prog1 (aref string q)
                   (aset string q (aref string p))))

  string)