more goodness dequeue

3.19.rkt

@@ -0,0 +1,43 @@
+#lang sicp
+
+;does structure contain cycle?
+
+(define (last-pair x)
+  (if (null? (cdr x))
+      x
+      (last-pair (cdr x))))
+
+(define (append! x y)
+  (set-cdr! (last-pair x) y)
+  x)
+
+
+(define (make-cycle x)
+  (set-cdr! (last-pair x) x)
+  x)
+
+(define z1 (make-cycle (list 'a 'b 'c 'a)))
+(define z2 (list (list 'a 'b 'c)(list 'a 'b 'c)(list 'a 'b 'c)))
+(define z3 (list 'a ))
+(append! (last-pair z3) z3)
+z1
+z2
+z3
+
+(define (contains-cycle? x)
+  (define (iter x seen)
+    (cond ((null? (cdr x)) #f)
+          ((memq x seen) #t)
+          (else
+           (begin
+             (set! seen (cons x seen))
+             (or (if (pair? (car x))
+                     (iter (car x) seen)
+                     #f)
+                 (iter (cdr x) seen))))))
+  (iter x '()))
+
+
+(contains-cycle? z1)
+(contains-cycle? z2)
+(contains-cycle? z3)