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

class Trie {
  TrieNode root;
  bool startWith = false;

  Trie() : root = TrieNode();

  // Inserts a word into the trie.
  void insert(String word) {
    _insert(word, root, 0);
  }

  void _insert(String word, TrieNode root, int idx) {
    if (idx == word.length) {
      root.isWord = true;
      return;
    }
    int index = word.codeUnitAt(idx) - 'a'.codeUnitAt(0);
    if (root.children[index] == null) {
      root.children[index] = TrieNode();
    }
    _insert(word, root.children[index]!, idx + 1);
  }

  // Returns if the word is in the trie.
  bool search(String word) {
    return _search(word, root, 0);
  }

  bool _search(String word, TrieNode root, int idx) {
    if (idx == word.length) {
      startWith = true;
      return root.isWord;
    }
    int index = word.codeUnitAt(idx) - 'a'.codeUnitAt(0);
    if (root.children[index] == null) {
      startWith = false;
      return false;
    }
    return _search(word, root.children[index]!, idx + 1);
  }

  // Returns if there is any word in the trie that starts with the given prefix.
  bool startsWith(String prefix) {
    _search(prefix, root, 0);
    return startWith;
  }
}

class TrieNode {
  List<TrieNode?> children;
  bool isWord;

  TrieNode()
      : children = List.filled(26, null),
        isWord = false;
}

/**
 * Your Trie object will be instantiated and called as such:
 * Trie obj = Trie();
 * obj.insert(word);
 * bool param2 = obj.search(word);
 * bool param3 = obj.startsWith(prefix);
 */

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