LeetCode 1804: Implement Trie II (Prefix Tree) Solution

Master LeetCode problem 1804 (Implement Trie II (Prefix Tree)), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

1804. Implement Trie II (Prefix Tree)

Medium

Hash Table String Design Trie

Problem Explanation

Explanation

A trie (pronounced "try") or prefix tree is a tree data structure that is used to store a dynamic set of strings. In this problem, we are required to implement a Trie II class that supports the following operations:

insert(word: str): Inserts a word into the trie.
countWordsEqualTo(word: str): Returns the number of words in the trie that are equal to the given word.
countWordsStartingWith(prefix: str): Returns the number of words in the trie that have the given prefix.

To implement this, we can use a TrieNode class to represent each node in the trie. Each node will have a map of character to the next TrieNode, a count of words that end at that node, and a count of words that start with the prefix leading to that node.

Solution Code

class TrieNode {
    Map<Character, TrieNode> children;
    int countEndsHere;
    int countStartsWith;

    public TrieNode() {
        children = new HashMap<>();
        countEndsHere = 0;
        countStartsWith = 0;
    }
}

class Trie {
    TrieNode root;

    public Trie() {
        root = new TrieNode();
    }

    public void insert(String word) {
        TrieNode curr = root;
        for (char c : word.toCharArray()) {
            curr.countStartsWith++;
            curr.children.putIfAbsent(c, new TrieNode());
            curr = curr.children.get(c);
        }
        curr.countEndsHere++;
    }

    public int countWordsEqualTo(String word) {
        TrieNode curr = root;
        for (char c : word.toCharArray()) {
            if (!curr.children.containsKey(c)) {
                return 0;
            }
            curr = curr.children.get(c);
        }
        return curr.countEndsHere;
    }

    public int countWordsStartingWith(String prefix) {
        TrieNode curr = root;
        for (char c : prefix.toCharArray()) {
            if (!curr.children.containsKey(c)) {
                return 0;
            }
            curr = curr.children.get(c);
        }
        return curr.countStartsWith;
    }
}

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