(medium)LeetCode .Implement Trie (Prefix Tree)

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：实现单词查找树，它是一种树形结构；用于保存大量的字符串。它的优点是：利用字符串的公共前缀来节约存储空间。每个节点有26个从节点。isEndOfWord,children，两个关键变量。

代码如下：

class TrieNode {
    // Initialize your data structure here.
    boolean isEndOfWord;
    TrieNode[] children;
    public TrieNode() {
        this.isEndOfWord=false;
        this.children=new TrieNode[26];
    }

}

public class Trie {
    private TrieNode root;

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

    // Inserts a word into the trie.
    public void insert(String word) {
        TrieNode runner=root;
        for(char c:word.toCharArray()){
            if(runner.children[c-'a']==null){
                runner.children[c-'a']=new TrieNode();
            }
            runner=runner.children[c-'a'];
        }
        runner.isEndOfWord=true;
    }

    // Returns if the word is in the trie.
    public boolean search(String word) {
       TrieNode runner=root;
       for(char c:word.toCharArray()){
           if(runner.children[c-'a']==null){
               return false;
           }else{
               runner=runner.children[c-'a'];
           }
       }
       return runner.isEndOfWord;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    public boolean startsWith(String prefix) {
        TrieNode runner=root;
        for(char c:prefix.toCharArray()){
            if(runner.children[c-'a']==null){
                return false;
            }else{
                runner=runner.children[c-'a'];
            }
        }
        return true;
    }
}

// Your Trie object will be instantiated and called as such:
// Trie trie = new Trie();
// trie.insert("somestring");
// trie.search("key");

　　运行结果：