My code:

class TrieNode {
    // Initialize your data structure here.
    char c;
    boolean isLeaf;
    HashMap<Character, TrieNode> tracker = new HashMap<Character, TrieNode>();
    public TrieNode() {
        
    }
}

public class Trie {
    private TrieNode root;

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

    // Inserts a word into the trie.
    public void insert(String word) {
        if (word == null || word.length() == 0)
            return;
        TrieNode temp = root;
        for (int i = 0; i < word.length(); i++) {
            char curr = word.charAt(i);
            if (temp.tracker.containsKey(curr)) {
                temp = temp.tracker.get(curr);
            }
            else {
                TrieNode newNode = new TrieNode();
                newNode.c = curr;
                temp.tracker.put(curr, newNode);
                temp = newNode;
            }
            if (i == word.length() - 1) {
                temp.isLeaf = true;
            }
        }
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        TrieNode ret = searchNode(word);
        if (ret == null || !ret.isLeaf)
            return false;
        else
            return true;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        TrieNode ret = searchNode(prefix);
        return ret == null ? false : true;
    }
    
    private TrieNode searchNode(String word) {
        if (word == null || word.length() == 0)
            return null;
        TrieNode temp = root;
        for (int i = 0; i < word.length(); i++) {
            char curr = word.charAt(i);
            if (!temp.tracker.containsKey(curr)) {
                return null;
            }
            else {
                temp = temp.tracker.get(curr);
            }
        }
        return temp;
    }
}

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

之前寫過trie。今天為了節省時間，直接看了答案再自己重寫了一遍。
http://www.programcreek.com/2014/05/leetcode-implement-trie-prefix-tree-java/

Anyway, Good luck, Richardo!

My code:

class TrieNode {
    // Initialize your data structure here.
    TrieNode[] next;
    char val;
    boolean isWord;
    public TrieNode() {
        next = new TrieNode[26];
    }
}

public class Trie {
    private TrieNode root;

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

    // Inserts a word into the trie.
    public void insert(String word) {
        insert(word, 0, root);
    }
    
    private void insert(String word, int index, TrieNode root) {
        char curr = word.charAt(index);
        TrieNode nextNode = null;
        if (root.next[curr -'a'] != null) {
            nextNode = root.next[curr - 'a'];
        }
        else {
            nextNode = new TrieNode();
            root.next[curr - 'a'] = nextNode;
        }
        if (index == word.length() - 1) {
            nextNode.isWord = true;
            return;
        }
        insert(word, index + 1, nextNode);
        
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
        TrieNode next = root;
        for (int i = 0; i < word.length(); i++) {
            char curr = word.charAt(i);
            if (next.next[curr - 'a'] == null) {
                return false;
            }
            else {
                next = next.next[curr - 'a'];
            }
        }
        return next.isWord;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        TrieNode next = root;
        for (int i = 0; i < prefix.length(); i++) {
            char curr = prefix.charAt(i);
            if (next.next[curr - 'a'] == null) {
                return false;
            }
            else {
                next = next.next[curr - 'a'];
            }
        }
        return true;
    }
}

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

差不多的思路吧。
reference:
https://leetcode.com/articles/implement-trie-prefix-tree/

Anyway, Good luck, Richardo! -- 09/11/2016