Medium | LeetCode 208. 实现 Trie (前缀树) | 设计

208. 实现 Trie (前缀树)

实现一个 Trie (前缀树)，包含 insert, search, 和 startsWith 这三个操作。

示例:

Trie trie = new Trie();

trie.insert("apple");
trie.search("apple");   // 返回 true
trie.search("app");     // 返回 false
trie.startsWith("app"); // 返回 true
trie.insert("app");   
trie.search("app");     // 返回 true

说明:

• 你可以假设所有的输入都是由小写字母 a-z 构成的。

• 保证所有输入均为非空字符串。

解题思路

class Trie {

    class TrieNode {
        private Map<Character, TrieNode> children = new HashMap<>();
        private String word = null;

        public TrieNode addChildren(Character ch) {
            TrieNode newNode = new TrieNode();
            children.put(ch, newNode);
            return newNode;
        }

        public void setWord(String word) {
            if (this.word != null) {
                return;
            }
            this.word = word;
        }

        public boolean hasChild(Character child) {
            return children.containsKey(child);
        }

        public TrieNode getChild(Character child) {
            return children.get(child);
        }
    }

    private TrieNode root;

    /** Initialize your data structure here. */
    public Trie() {
        this.root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode p = root;
        for (Character ch : word.toCharArray()) {
            if (!p.hasChild(ch)) {
                p = p.addChildren(ch);
            } else {
                p = p.getChild(ch);
            }
        }
        p.setWord(word);
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode p = root;
        for (Character ch : word.toCharArray()) {
            if (p.hasChild(ch)) {
                p = p.getChild(ch);
            } else {
                return false;
            }
        }
        return p.word != null;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode p = root;
        for (Character ch : prefix.toCharArray()) {
            if (p.hasChild(ch)) {
                p = p.getChild(ch);
            } else {
                return false;
            }
        }
        return true;
    }
}