[LeetCode] 208. Implement Trie (Prefix Tree)

Problem

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 * 10^4 calls in total will be made to insert, search, and startsWith.

---

Solution

class Trie {
    List<String> list;

    public Trie() {
        this.list = new ArrayList<>();
    }
    
    public void insert(String word) {
        list.add(word);
    }
    
    public boolean search(String word) {
        return list.contains(word);
    }
    
    public boolean startsWith(String prefix) {
        for (int i = 0; i < list.size(); i++)
            if (list.get(i).startsWith(prefix))
                return true;
        return false;
    }
}

/**
 * Your Trie object will be instantiated and called as such:
 * Trie obj = new Trie();
 * obj.insert(word);
 * boolean param_2 = obj.search(word);
 * boolean param_3 = obj.startsWith(prefix);
 */

Explanation

• 문자열을 관리하기 위한 자료구조가 필요한데, 문제를 빠르게 해결하고자 단순히 List를 선택하였다. 그리고 자료구조 List와 String 클래스의 메서드를 활용해서 문제를 해결하였다.

  class Trie {
    List<String> list;
  
    public Trie() {
        this.list = new ArrayList<>();
    }
      
    public void insert(String word) {
        list.add(word);
    }
      
    public boolean search(String word) {
        return list.contains(word);
    }
      
    public boolean startsWith(String prefix) {
        for (int i = 0; i < list.size(); i++)
            if (list.get(i).startsWith(prefix))
                return true;
        return false;
    }
  }