【Leetcode】Implement Trie (Prefix Tree)

题目链接：https://leetcode.com/problems/implement-trie-prefix-tree/
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.

思路：

每个结点包括三个属性：结点代表的字符、指向儿子结点的指针、代表该结点是否是word。最后一个属性是因为word不一定是从根到叶结点的路径。比如insert("abc")、insert("ab")，search("ab")应该返回true。因为之前插入了ab，哪怕b不是叶结点，所以结点要有一个属性表示从root到该是否为word。

算法：

 import java.util.HashMap;

 class TrieNode {
     // Initialize your data structure here.
     String val = "";
     HashMap<String, TrieNode> nexts = new HashMap<String, TrieNode>();
     boolean isword = false;
     public TrieNode() {
     }
 }

 public class Trie {
     private TrieNode root;

     public Trie() {
         root = new TrieNode();
     }

     // Inserts a word into the trie.
     public void insert(String word) {
         TrieNode p = root;
         for (int i = 0; i < word.length(); i++) {
             String key = word.charAt(i) + "";
             if (p.nexts.containsKey(key)) {
                 p = p.nexts.get(key);
             } else {
                 TrieNode t = new TrieNode();
                 t.val = key;
                 p.nexts.put(key, t);
                 p = t;
             }
             if(i==word.length()-1){
                 p.isword = true;
             }
         }
         System.out.println(p.val+":"+p.isword);
     }

     // Returns if the word is in the trie.
     public boolean search(String word) {
         TrieNode p = root;
         for (int i = 0; i < word.length(); i++) {
             String key = word.charAt(i) + "";
             if (p.nexts.containsKey(key)) {
                 p = p.nexts.get(key);
             } else {
                 return false;
             }
         }
         if (p.nexts.size() == 0)
             return true;
         else if(!p.isword)
             return false;
         else
             return true;
     }

     // Returns if there is any word in the trie
     // that starts with the given prefix.
     public boolean startsWith(String prefix) {
         TrieNode p = root;
         for (int i = 0; i < prefix.length(); i++) {
             String key = prefix.charAt(i) + "";
             if (p.nexts.containsKey(key)) {
                 p = p.nexts.get(key);
             } else {
                 return false;
             }
         }
         return true;
     }
 }