LeetCode：208.实现Trie（前缀树）

字典树的结构图：

字典树有三个特点：

1、根节点不包含字符，除根节点外每一个节点都只包含一个字符

2、 从根节点到某一节点，路径上经过的字符连接起来，为该节点对应的字符串

3、每个节点的所有子节点包含的字符都不相同

首先定义字典树的节点：

Class Node {

    boolean isWord;
    Map next;

}

isWord标识从根节点到当前节点组成的字符串是不是一个单词

每个节点维护了一个Map，根据字符可以找到下一个Node

public class Trie {

	class Node {
		boolean isWord;
		Map next;

		Node() {
			isWord = false;
			next = new HashMap<>();
		}

		boolean containsKey(char key) {
			return next.containsKey(key);
		}

		void put(char key) {
			if (!next.containsKey(key)) {
				next.put(key, new Node());
			}
		}

		Node get(char key) {
			return next.get(key);
		}

		boolean isWord() {
			return isWord;
		}

		void setIsWord() {
			isWord = true;
		}
	}

	private Node root;

	/** Initialize your data structure here. */
	public Trie() {
		root = new Node();
	}

	/** Inserts a word into the trie. */
	public void insert(String word) {
		Node current = root;
		char[] chs = word.toCharArray();
		for (int i = 0; i < chs.length; i++) {
			if (!current.containsKey(chs[i])) {
				current.put(chs[i]);
			}
			current = current.get(chs[i]);
		}
		current.setIsWord();
	}

	/** Returns if the word is in the trie. */
	public boolean search(String word) {
		Node node = searchPre(word);
		return node == null ? false : node.isWord;
	}

	/**
	 * Returns if there is any word in the trie that starts with the given
	 * prefix.
	 */
	public boolean startsWith(String prefix) {
		return searchPre(prefix) != null;
	}

	private Node searchPre(String word) {
		Node current = root;
		char[] chs = word.toCharArray();
		for (int i = 0; i < chs.length; i++) {
			if (current.containsKey(chs[i])) {
				current = current.get(chs[i]);
			} else {
				return null;
			}
		}
		return current;
	}
}

由于题目中给出可以假设所有输入中只包含小写a-z的字符，所以可以使用数组来替代Map，ch - 'a'即是数组的下标