Data commit

Task/Permutations-by-swapping/Racket/permutations-by-swapping.rkt

@@ -0,0 +1,20 @@
+#lang racket
+
+(define (add-at l i x)
+  (if (zero? i) (cons x l) (cons (car l) (add-at (cdr l) (sub1 i) x))))
+
+(define (permutations l)
+  (define (loop l)
+    (cond [(null? l) '(())]
+          [else (for*/list ([(p i) (in-indexed (loop (cdr l)))]
+                            [i ((if (odd? i) identity reverse)
+                                (range (add1 (length p))))])
+                  (add-at p i (car l)))]))
+  (for/list ([p (loop (reverse l))] [i (in-cycle '(1 -1))]) (cons i p)))
+
+(define (show-permutations l)
+  (printf "Permutations of ~s:\n" l)
+  (for ([p (permutations l)])
+    (printf "  ~a (~a)\n" (apply ~a (add-between (cdr p) ", ")) (car p))))
+
+(for ([n (in-range 3 5)]) (show-permutations (range n)))