Permutations: Difference between revisions

Completely recursive on lists:

<lang lisp>(define (perm s)

(cond ((null? s) '())

((null? (cdr s)) (list s))

(else ;; extract each item in list in turn and perm the rest

(let splice ((l '()) (m (car s)) (r (cdr s)))

(append

(map (lambda (x) (cons m x)) (perm (append l r)))

(if (null? r) '()

(splice (cons m l) (car r) (cdr r))))))))

(display (perm '(1 2 3)))</lang>