Java for LeetCode 208 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.

解题思路：

参考百度百科：Trie树

已经给出了详细的代码：

JAVA实现如下：

class TrieNode {
	// Initialize your data structure here.
	int num;// 有多少单词通过这个节点,即节点字符出现的次数
	TrieNode[] son;// 所有的儿子节点
	boolean isEnd;// 是不是最后一个节点
	char val;// 节点的值

	TrieNode() {
		this.num = 1;
		this.son = new TrieNode[26];
		this.isEnd = false;
	}
}

public class Trie {
	private TrieNode root;

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

	public void insert(String word) {
		if (word == null || word.length() == 0)
			return;
		TrieNode node = this.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 == null || word.length() == 0) {
			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) {
				node = node.son[pos];
			} else {
				return false;
			}
		}
		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) {
				node = node.son[pos];
			} else {
				return false;
			}
		}
		return true;
	}
}

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