Leetcode 208. Implement Trie (Prefix Tree)

方法1: hashset。这是我能想到的方法。

class Trie {
    Set<String> set = new HashSet<>();
    /** Initialize your data structure here. */
    public Trie() {
        
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        set.add(word);
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        return set.contains(word);
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        int len = prefix.length();
        for(String s : set){
            if(s.length() >= len)
                if(s.substring(0, len).equals(prefix)) return true;
        }
        return false;
    }
}

/**
 * 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);
 */

方法2: lc官方解答1。这个方法思路是参考的lc官方解答1，但是代码是按照这个帖子写的。这是个用一个数组实现trie的算法，非常巧妙。具体思路请参考lc官方解答1.

class TrieNode {
    public char val;
    public boolean isEnd; 
    public TrieNode[] children = new TrieNode[26];
    public TrieNode() {}
    TrieNode(char c){
        TrieNode node = new TrieNode();
        node.val = c;
    }
}

class Trie {
    TrieNode root;
    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
        root.val = ' ';
    }
    
    /** 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.children[c - 'a'] == null){
                node.children[c - 'a'] = new TrieNode(c);
            }
            node = node.children[c - 'a'];
        }
        node.isEnd = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode node = root;
        for(int i = 0; i < word.length(); i++){
            char c = word.charAt(i);
            if(node.children[c - 'a'] == null) return false;
            node = node.children[c - 'a'];
        }
        return node.isEnd;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode node = root;
        for(int i = 0; i < prefix.length(); i++){
            char c = prefix.charAt(i);
            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);
 */

总结：

• 无