208. Implement Trie (Prefix Tree)

class TrieNode {
    
    public HashMap<Character, TrieNode> children = new HashMap<>();
    public boolean exist = false;
    
    public TrieNode() {
        
    }
}

public class Trie {
    private TrieNode root;

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

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

    // Returns if the word is in the trie.
    public boolean search(String word) {
        TrieNode r = root;
        for(int i = 0; i<word.length(); ++i)
        {
            Character c = word.charAt(i);
            TrieNode child = r.children.get(c);
            if(child == null)
                return false;
            r = child;
        }
        return r.exist;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        TrieNode r = root;
        for(int i = 0; i<prefix.length(); ++i)
        {
            Character c = prefix.charAt(i);
            TrieNode child = r.children.get(c);
            if(child == null)
                return false;
            r = child;
        }
        return true;
    }
}

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