LeetCode-in-All

208. Implement Trie (Prefix Tree)

Medium

A trie (pronounced as “try”) or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker.

Implement the Trie class:

• Trie() Initializes the trie object.
• void insert(String word) Inserts the string word into the trie.
• boolean search(String word) Returns true if the string word is in the trie (i.e., was inserted before), and false otherwise.
• boolean startsWith(String prefix) Returns true if there is a previously inserted string word that has the prefix prefix, and false otherwise.

Example 1:

Input [“Trie”, “insert”, “search”, “search”, “startsWith”, “insert”, “search”] [[], [“apple”], [“apple”], [“app”], [“app”], [“app”], [“app”]]

Output: [null, null, true, false, true, null, true]

Explanation:

Trie trie = new Trie();

trie.insert(“apple”);

trie.search(“apple”); // return True

trie.search(“app”); // return False

trie.startsWith(“app”); // return True

trie.insert(“app”);

trie.search(“app”); // return True

Constraints:

• 1 <= word.length, prefix.length <= 2000
• word and prefix consist only of lowercase English letters.
• At most 3 * 104 calls in total will be made to insert, search, and startsWith.

Solution

(define trie%
  (class object%
    (super-new)

    ;; Define TrieNode struct
    (struct trie-node (children is-word?) #:mutable)

    ;; Root node of the Trie
    (init-field)
    (define root (trie-node (make-hash) #f))
    (define start-with? #f)

    ;; Inserts a word into the trie.
    (define/public (insert word)
      (define (insert-helper node word idx)
        (if (= idx (string-length word))
            (set-trie-node-is-word?! node #t)
            (let* ([ch (string-ref word idx)]
                   [children (trie-node-children node)]
                   [next-node (hash-ref children ch (lambda () (trie-node (make-hash) #f)))])
              (hash-set! children ch next-node)
              (insert-helper next-node word (+ idx 1)))))
      (insert-helper root word 0))

    ;; Searches for a word in the trie.
    (define/public (search word)
      (define (search-helper node word idx)
        (if (= idx (string-length word))
            (begin
              (set! start-with? #t)
              (trie-node-is-word? node))
            (let* ([ch (string-ref word idx)]
                   [children (trie-node-children node)]
                   [next-node (hash-ref children ch #f)])
              (if next-node
                  (search-helper next-node word (+ idx 1))
                  (begin
                    (set! start-with? #f)
                    #f)))))
      (search-helper root word 0))

    ;; Checks if any word in the trie starts with the given prefix.
    (define/public (starts-with prefix)
      (send this search prefix)
      start-with?)))

;; Your trie% object will be instantiated and called as such:
;; (define obj (new trie%))
;; (send obj insert word)
;; (define param_2 (send obj search word))
;; (define param_3 (send obj starts-with prefix))

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