PrefixTree.java

import java.util.HashMap;

class TrieNode {
    char letter;
    HashMap<Character, TrieNode> children;
        
    public TrieNode(char letter) {
        this.letter = letter;
        children = new HashMap<Character, TrieNode>();
    }
    
    public void addChild(TrieNode child) {
        children.put(child.letter, child);
    }
}

public class Trie {
    private TrieNode root;

    public Trie() {
        root = new TrieNode('0');
    }

    // Inserts a word into the trie.
    public void insert(String word) {
        TrieNode currentNode = root;
        
        for (int i=0; i<word.length(); i++) {
            char c = word.charAt(i);
            
            if (currentNode.children.containsKey(c)) {
                currentNode = currentNode.children.get(c);
            } else {
                TrieNode newNode = new TrieNode(c);
                currentNode.addChild(newNode);
                currentNode = newNode;
            }
        }
        
        currentNode.addChild(new TrieNode('*'));
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        return search(word, false);
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        return search(prefix, true);
    }
    
    private boolean search(String word, boolean prefix) {
        TrieNode currentNode = root;
        
        for (int i=0; i<word.length(); i++) {
            char c = word.charAt(i);
            
            if (currentNode.children.containsKey(c)) {
                currentNode = currentNode.children.get(c);
            } else {
                return false;
            }
        }
        
        return prefix || (!prefix && currentNode.children.containsKey('*'));
    }
}

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