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.

class Trie {
    private int SIZE = 26;
    private TrieNode root;
    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }
    
    private class TrieNode{
        private int num;
        private TrieNode[] son;
        private boolean isEnd;
        private char val;
        private boolean haveSon;
        
        TrieNode(){
            num = 1;
            son = new TrieNode[SIZE];
            isEnd = false;
            haveSon = false;
        }
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        if(word.length() == 0 || word == null) return;
        TrieNode node = root;
        char[] letters = word.toCharArray();
        for(int i = 0; i < word.length(); i++){
            int pos = letters[i] - 'a';
            if(node.son[pos] == null){
                node.son[pos] = new TrieNode();
                node.son[pos].val = letters[i];
            }
            else node.son[pos].num++;
            node = node.son[pos];
        }
        node.isEnd = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        if(word.length() == 0 || word == null) return false;
        TrieNode node = root;
        char[] letters = word.toCharArray();
        for(int i = 0; i < word.length();i++){
            int pos = letters[i] - 'a';
            if(node.son[pos] == null) return false;
            else node = node.son[pos];
        }
        return node.isEnd;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        if(prefix == null || prefix.length() == 0) return false;
        TrieNode node = root;
        char[] letters = prefix.toCharArray();
        for(int i = 0; i < prefix.length(); i++){
            int pos = letters[i] - 'a';
            if(node.son[pos] == null) return false;
            else node = node.son[pos];
        }
        return true;
    }
}

很有意义的题目，prefix tree，root不包含任何val。

https://baike.baidu.com/item/%E5%AD%97%E5%85%B8%E6%A0%91/9825209?fr=aladdin

详细讲解在这里。

一个trie里面应该要有TrieNode，TrieNode里面有son[], num, val, haveSon, isEnd，其中son是儿子数组，num是由根经过此点的单词数量，isEnd是是否到底了。

然后是各种method，无论是insert还是search都要从root开始，所以初始化都要先把root提出来，insert如果当前char不存在，就新建一个trieNode。如果存在就num++。

search更简单，从头到尾找有没有对应的TrieNode，最后返回是不是到底了。找prefix是search的简单版，如果中间没有任何差错，遍历完就可以返回true。