LeetCode in Elixir

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

defmodule Trie do
  use GenServer

  @spec init_() :: any
  def init_() do
    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
    GenServer.call(__MODULE__, :start)
  end

  @spec insert(word :: String.t) :: any
  def insert(word) do
    GenServer.call(__MODULE__, {:insert, word})
  end

  @spec search(word :: String.t) :: boolean
  def search(word) do
    GenServer.call(__MODULE__, {:search, word})
  end

  @spec starts_with(prefix :: String.t) :: boolean
  def starts_with(prefix) do
    GenServer.call(__MODULE__, {:prefix, prefix})
  end

  @impl true
  def init(_) do
    {:ok, nil}
  end

  @impl true
  def handle_call(:start, _, _) do
    {:reply, nil, Map.new()}
  end

  def handle_call({:insert, word}, _, map) do
    String.codepoints(word)
    |> Enum.reduce({map, ""}, fn ch, {map, prefix} ->
      {Map.put_new(map, prefix, false), prefix <> ch}
    end)
    |> elem(0)
    |> Map.put(word, true)
    |> then(fn map -> {:reply, nil, map} end)
  end

  def handle_call({:search, word}, _, map) do
    {:reply, Map.get(map, word, false), map}
  end

  def handle_call({:prefix, prefix}, _, map) do
    {:reply, Map.get(map, prefix) != nil, map}
  end
end

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