Implement Trie (Prefix Tree) | Leetcode

Implement a trie with insert, search, and startsWith methods.

Note:
You may assume that all inputs are consist of lowercase letters a-z.

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Solution: The main thing here is really the data structure to store the trie. I found that using a map of character to TrieNode is sufficient to represent the structure. The other parts of the code only consist of single for loops.

class TrieNode {
    // Initialize your data structure here.
    Map<Character, TrieNode> children = new HashMap<>();
}

public class Trie {
    private static final char TERMINAL = (char) -1;
    private TrieNode root = new TrieNode();

    // Inserts a word into the trie.
    public void insert(String word) {
        TrieNode node = root;
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            TrieNode child = node.children.get(c);
            if (child == null) {
                child = new TrieNode();
                node.children.put(c, child);
            }
            node = child;
        }
        node.children.put(TERMINAL, node);
    }

    private TrieNode searchPrefix(String prefix) {
        TrieNode node = root;
        for (int i = 0; i < prefix.length(); i++) {
            char c = prefix.charAt(i);
            node = node.children.get(c);
            if (node == null) return null;
        }
        return node;
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        TrieNode node = searchPrefix(word);
        if (node == null) return false;
        return node.children.containsKey(TERMINAL);
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        TrieNode node = searchPrefix(prefix);
        return (node != null);
    }
}

// Your Trie object will be instantiated and called as such:
// Trie trie = new Trie();
// trie.insert("somestring");
// trie.search("key");