[LeetCode] 208. Implement Trie (Prefix Tree) Java

题目：

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

Note:
You may assume that all inputs are consist of lowercase letters a-z.

题意及分析：实现一个字典树或者叫前缀树的插入、删除和startsWith判断。这里定义一个trieNode类作为字典树的节点类。然后有一个chilrden数组保存子节点，一个isWord变量来判断从根节点到该节点是否是一个单词。这里我用的是长度为26的数组来保存子节点，浪费了一些内存，也可以用一个hashmap来实现。

代码：

class TrieNode{
    public char word;
    boolean isWord;     //记录是否是单词结尾
    public TrieNode[] childNodes = new TrieNode[26];
    public TrieNode(){};
    public TrieNode(char c){
        TrieNode node = new TrieNode();
        node.word = c;
    }
}

public class Trie {

    private TrieNode root;
    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
        root.word = ‘ ‘;
    }

    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode node = root;
        for(int i=0;i<word.length();i++){
            if(node.childNodes[(word.charAt(i)-‘a‘)]!=null){
                node = node.childNodes[(word.charAt(i)-‘a‘)];
            }else{      //后面的需要重新创建
                for(int j=i;j<word.length();j++){       //重新创建一颗子树
                    node.childNodes[(word.charAt(j)-‘a‘)] = new TrieNode(word.charAt(j));
                    node = node.childNodes[(word.charAt(j)-‘a‘)];
                }
                break;  //跳出循环
            }
        }
        node.isWord = true;
    }

    /** Returns if the word is in the trie. */
    public boolean search(String word) {        //要到根节点才能判断，中间的不算
        TrieNode node = root;
        for(int i=0;i<word.length();i++) {
            if (node.childNodes[(word.charAt(i) - ‘a‘)] != null) {
                node = node.childNodes[(word.charAt(i) - ‘a‘)];
            }else {
                return false;
            }
        }
        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(int i=0;i<prefix.length();i++) {
            if (node.childNodes[(prefix.charAt(i) - ‘a‘)] != null) {
                node = node.childNodes[(prefix.charAt(i) - ‘a‘)];
            }else {
                return false;
            }
        }
        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);
 */