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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

# @param {String} string
# @return {String}
class Trie
  def initialize
    @root = TrieNode.new
    @start_with = false
  end

=begin
    :type word: String
    :rtype: Void
=end
  def insert(word)
    insert_recursive(word, @root, 0)
  end

=begin
    :type word: String
    :rtype: Boolean
=end
  def search(word)
    search_recursive(word, @root, 0)
  end

=begin
    :type prefix: String
    :rtype: Boolean
=end
  def starts_with(prefix)
    search(prefix)
    @start_with
  end

  private

  class TrieNode
    attr_accessor :children, :is_word

    def initialize
      @children = Array.new(26)
      @is_word = false
    end
  end

  def insert_recursive(word, root, idx)
    if idx == word.length
      root.is_word = true
      return
    end

    index = word[idx].ord - 'a'.ord
    root.children[index] ||= TrieNode.new
    insert_recursive(word, root.children[index], idx + 1)
  end

  def search_recursive(word, root, idx)
    if idx == word.length
      @start_with = true
      return root.is_word
    end

    index = word[idx].ord - 'a'.ord
    if root.children[index].nil?
      @start_with = false
      return false
    end

    search_recursive(word, root.children[index], idx + 1)
  end
end

# Your Trie object will be instantiated and called as such:
# obj = Trie.new()
# obj.insert(word)
# param_2 = obj.search(word)
# param_3 = obj.starts_with(prefix)

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