Updated.

trees/main.rkt

@@ -1,7 +1,7 @@
 ;; The first three lines of this file were inserted by DrRacket. They record metadata
 ;; about the language level of this file in a form that our tools can easily process.
 #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
+; A MaybeTree is one of
 ; - (make-tree [Number MaybeTree MaybeTree])
 (define-struct tree [n left right])
 ; - 'end
@@ -41,7 +41,6 @@
 (check-expect (bst? sapling) #t)
 (check-expect (bst? good-tree) #t)
 (check-expect (bst? broke-tree) #f)
-|#
 
 ; A Trie is a [Node]
 
@@ -90,6 +89,26 @@
   (local [; String w, Trie t -> Trie
           ; Adds word w to a Trie.
           (define (triadd w t)
-            (if (
+            (local [; String c, String wrest, Trie t -> Trie
+                    ; Helper function to insert a chacater (and the rest of the word) to the Trie.
+                    (define (insert-char c wrest t)
+                      (cond [(empty? t)
+                             (list (make-node c ; Node character.
+                                              (string=? wrest "") ; Mark whether Node's a word end.
+                                              (if (string=? wrest "") ; If more left in the word, recurse.
+                                                  '()
+                                                  (triadd wrest '()))))]
+                            [(string=? (node-c (first t)) c) ; Character matches -- return existing Trie Node + whatever added if necessary.
+                             (cons (make-node c
+                                              (or (node-e (first t))
+                                                  (string=? wrest ""))
+                                              (if (string=? wrest "")
+                                                  (node-l (first t))
+                                                  (triadd wrest (node-l (first t))))) ; Recurse.
+                                   (rest t))]
+                            [else
+                             (cons (first t) (insert-char c wrest (rest t)))]))]
+              (insert-char (substring w 0 1) (substring w 1) t)))]
+    (foldl triadd '() l)))
 
-(check-exepct (strie '("and" "bee" "cat" "an" "a" "ant" "better" "bet" "be")) example-trie)
+(check-expect (strie '("and" "bee" "cat" "an" "a" "ant" "better" "bet" "be")) example-trie)