Clojure: How to generate a 'trie'?

Question

Given the following...

(def inTree
 \'((1 2)
   (1 2 3)
   (1 2 4 5 9)
   (1 2 4 10 15)
   (1 2 4 20 25)))

How would you transform it to this t

Answer 1

Lists are very clumsy here, not to mention inefficient. In Clojure it's more idiomatic to use vectors and hash-maps and sets when appropriate. Using hash-maps:

(def in-tree
 '((1 2)
   (1 2 3)
   (1 2 4 5 9)
   (1 2 4 10 15)
   (1 2 4 20 25)))

(defn add-to-trie [trie x]
  (assoc-in trie `(~@x :terminal) true))

(defn in-trie? [trie x]
  (get-in trie `(~@x :terminal)))

If you wanted it to print sorted you could use sorted-maps instead, but you'd have to write your own version of assoc-in that used sorted maps the whole way down. In any case:

user> (def trie (reduce add-to-trie {} in-tree))
#'user/trie
user> trie
{1 {2 {4 {20 {25 {:terminal true}}, 10 {15 {:terminal true}}, 5 {9 {:terminal true}}}, 3 {:terminal true}, :terminal true}}}
user> (in-trie? trie '(1 2))
true
user> (in-trie? trie '(1 2 4))
nil
user> (in-trie? trie '(1 2 4 20 25))
true