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 * 104 calls in total will be made to insert, search, and startsWith.

Solution

This problem is asking us to implement a very basic Trie.

Trie

class Trie {
    class TrieNode {
        private Map<Character, TrieNode> children = new HashMap<>();
        private boolean terminates = false;

        TrieNode() {
        }

        void insert(String word) {
            if (word == null || word.isEmpty()) return;

            char firstChar = word.charAt(0);
            TrieNode child = children.get(firstChar);
            if (child == null) {
                child = new TrieNode();
                children.put(firstChar, child);
            }

            if (word.length() > 1) {
                child.insert(word.substring(1));
            } else {
                child.terminates = true;
            }
        }

        TrieNode getChild(char c) {
            return children.get(c);
        }

        boolean terminates() {
            return terminates;
        }
    }

    private TrieNode root = new TrieNode();

    public Trie() {
    }
    
    public void insert(String word) {
        root.insert(word);
    }
    
    public boolean search(String word) {
        return search(word, true);
    }
    
    public boolean startsWith(String prefix) {
        return search(prefix, false);
    }

    private boolean search(String word, boolean exact) {
        TrieNode lastNode = root;
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            lastNode = lastNode.getChild(c);
            if (lastNode == null) return false;
        }
        return !exact || lastNode.terminates();
    }
}

Reference

• 208. Implement Trie (Prefix Tree), LeetCode.
• 208. Implement Trie (Prefix Tree), LeetCode Solutions.