More.

trees/main.rkt~

@@ -0,0 +1,74 @@
+#reader(lib "htdp-intermediate-reader.ss" "lang")((modname main) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))
+
+#|
+; A MaybeTree is one of
+; - (make-tree [Number MaybeTree MaybeTree])
+(define-struct tree [n left right])
+; - 'end
+
+(define sprout (make-tree 0 'end 'end))
+(define seedling (make-tree 5 sprout (make-tree 6 'end 'end)))
+(define sapling (make-tree 10
+                           (make-tree 6 sprout 'end)
+                           (make-tree 12 'end 'end)))
+(define good-tree (make-tree 50 sapling (make-tree 100 'end 'end)))
+(define broke-tree (make-tree 5 good-tree good-tree))
+
+; Tree t -> Bool
+; Determines whether a Tree is a binary search tree.
+(define (bst? t)
+  (local [; Tree t -> Bool
+          ; Recursively checks left and right.
+          (define (recurse t)
+            (or (and (symbol? t) (symbol=? t 'end))
+                (and (acc-right (tree-right t) (tree-n t))
+                     (acc-left (tree-left t) (tree-n t)))))
+
+          ; Tree t, Number n -> Bool
+          ; Accumulates the current number to check left.
+          (define (acc-left t n)
+            (or (and (symbol? t) (symbol=? t 'end))
+                (and (< (tree-n t) n) (recurse t))))
+
+          ; Tree t, Number n -> Bool
+          ; Accumulates the current number to check right.
+          (define (acc-right t n)
+            (or (and (symbol? t) (symbol=? t 'end))
+                (and (> (tree-n t) n) (recurse t))))]
+    (recurse t)))
+
+(check-expect (bst? sprout) #t)
+(check-expect (bst? sapling) #t)
+(check-expect (bst? good-tree) #t)
+(check-expect (bst? broke-tree) #f)
+|#
+
+; A Trie is a [Node]
+
+; A Node is a
+; (make-node [String c Boolean e [Node] l])
+; and represents a prefix tree. 'c' represents the node character, 'e' is a
+; flag for if the node marks the end of a word in the tree, and 'l' is the list
+; of child nodes.
+(define-struct node [c e l])
+
+(define example-trie
+  '((make-node "a" #t
+               ((make-node "n"  #t ((make-node "d" #t ()) (make-node "t" #t ())))))
+    (make-node "b" #f
+               ((make-node "e" #t ((make-node "e" #t ())
+                                   (make-node "t" #t ((make-node "t" #f ((make-node "e" #f ((make-node "r" #t ())))))))))))
+    (make-node "c" #f ((make-node "a" #f ((make-node "t" #t ())))))))
+
+; String w, Trie t -> Boolean
+; Determines whether the given word is in the given Trie.
+(define (in-trie? w t)
+  (local [; [String])
+
+(check-expect (in-trie? "cat" example-trie) #t)
+(check-expect (in-trie? "ca" example-trie) #f)
+(check-expect (in-trie? "catch" example-trie) #f)
+(check-expect (in-trie? "better" example-trie) #t)
+(check-expect (in-trie? "bee" example-trie) #t)
+(check-expect (in-trie? "beee" example-trie) #f)
+