208. Implement Trie

Problem:

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

6/5/2018 update:
现在倾向于用hash map来表示children.

 class TrieNode {
        Map<Character, TrieNode> children;
        boolean isWord;
        public TrieNode() {
            children = new HashMap<>();
        }
    }
    TrieNode root;
    /** Initialize your data structure here. */
    public WordDictionary() {
        root = new TrieNode();
    }
    
    /** Adds a word into the data structure. */
    public void addWord(String word) {
        TrieNode cur = root;
        for (char c: word.toCharArray()) {
            if (!cur.children.containsKey(c)) {
                cur.children.put(c, new TrieNode()); 
            } 
            cur = cur.children.get(c);
        }
        cur.isWord = true;
    }

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Analysis:

第一次接触trie的时候，还消化了挺久。经过几个月的刷题，功力大增。第二次实现trie，秒过啊。而且还优化了实现方法。

Trie node is the key to trie tree. 

Solution:

class Trie {
    class TrieNode {
        char val;
        TrieNode[] children;
        boolean isWord;
        public TrieNode() {}
        public TrieNode(char val) {
            this.val = val;
            children = new TrieNode[26];
        }
    }
    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 (char c: word.toCharArray()) {
            if (node.children[c - 'a'] == null) {
                node.children[c - 'a'] = new TrieNode(c);
            }
            node = node.children[c - 'a'];
        }
        node.isWord = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode node = root;
        for (char c: word.toCharArray()) {
            if (node.children[c - 'a'] == null) {
               return false;
            }
            node = node.children[c - 'a'];
        }
        return node.isWord;   
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode node = root;
        for (char c: prefix.toCharArray()) {
            if (node.children[c - 'a'] == null) {
               return false;
            }
            node = node.children[c - 'a'];
        }
        return true;    
    }
}

/**
 * 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);
 */

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 /**  
  * Your Trie object will be instantiated and called as such:  
  * Trie trie = new Trie();  
  * trie.insert("lintcode");  
  * trie.search("lint"); will return false  
  * trie.startsWith("lint"); will return true  
  */  
 class TrieNode {  
   // Initialize your data structure here.  
   private TrieNode[] links;  
   private final int R=26;  
   private boolean isEnd;  
   public TrieNode() {  
     links=new TrieNode[R];  
   }  
   public boolean containsKey(char c)  
   {  
     return links[c-'a']!=null;  
   }  
   public TrieNode get(char c)  
   {  
     return links[c-'a'];  
   }  
   public void put(char c,TrieNode t)  
   {  
     links[c-'a']=t;  
   }  
   public void setEnd()  
   {  
     isEnd=true;  
   }  
   public boolean isEnd()  
   {  
     return isEnd;  
   }  
 }  
 public class Trie {  
   private TrieNode root;  
   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 c=word.charAt(i);  
       if(!node.containsKey(c))  
       {  
         node.put(c,new TrieNode());  
       }  
       node=node.get(c);  
     }  
     node.setEnd();  
   }  
   private TrieNode searchPrefix(String word)  
   {  
     TrieNode node=root;  
     for(int i=0;i<word.length();i++)  
     {  
       char c=word.charAt(i);  
       if(!node.containsKey(c))  
       {  
         return null;  
       }  
       node=node.get(c);  
     }  
     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;  
   }  
 }