LC208.java

/*
208. Implement Trie (Prefix Tree)

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

Example:

Trie trie = new Trie();

trie.insert("apple");
trie.search("apple");   // returns true
trie.search("app");     // returns false
trie.startsWith("app"); // returns true
trie.insert("app");   
trie.search("app");     // returns true
Note:

You may assume that all inputs are consist of lowercase letters a-z.
All inputs are guaranteed to be non-empty strings.
*/

/**
 * Your Trie object will be instantiated and called as such:
 * Trie obj = new Trie();
 * obj.insert(word);
 * boolean param_2 = obj.search(word);
 * boolean param_3 = obj.startsWith(prefix);
 */

class Trie {

    class TrieNode
    {
        private TrieNode[] links;
        private final int R = 26;
        private boolean isEnd;
        
        public TrieNode()
        {
            links = new TrieNode[R];
        }
        
        public boolean containsKey(char ch)
        {
            return links[ch-'a'] != null;
        }
        
        public TrieNode get(char ch)
        {
            return links[ch-'a'];
        }
        public void put(char ch, TrieNode node)
        {
            links[ch-'a'] = node;
        }
        public void setEnd()
        {
            isEnd = true;
        }
        public boolean isEnd()
        {
            return isEnd;
        }
    }
    
    private TrieNode root;
    /** Initialize your data structure here. */
    public Trie() {
        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 currChar = word.charAt(i);
            
            if (!node.containsKey(currChar))
            {
                node.put(currChar, new TrieNode());
            }
            
            node = node.get(currChar);
        }
        
        node.setEnd();
    }
    
    private TrieNode searchPrefix(String word)
    {
        TrieNode node = root;
        
        for (int i = 0; i < word.length(); i++)
        {
            char currLetter = word.charAt(i);
            
            if (node.containsKey(currLetter))
            {
                node = node.get(currLetter);
            }
            else
                return null;
        }
        
        return node;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) 
    {
        TrieNode node = searchPrefix(word);
        return node != null && node.isEnd();
    }
    
    /** 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;
    }
}