leetcode 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。

思路:
Trie的基本实现,两种解题方法,一种是递归实现,一种是迭代实现
递归实现:

package Trie;

import java.util.TreeMap;

// leetcode 208. 实现 Trie (前缀树)
class Trie {

    private class Node {
        public boolean isWord;
        public TreeMap<Character, Node> next;

        public Node(boolean isWord) {
            this.isWord = isWord;
            next = new TreeMap<>();
        }

        public Node() {
            this(false);
        }
    }

    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 cur = root;
        add(root, word, 0);
    }

    public void add(Node node, String word, int index) {
        // 此时一个单词已经遍历完成了,如果这个结束节点没有标记为单词,就标记为单词
        if (!node.isWord && index == word.length()) {
            node.isWord = true;
        }

        if (word.length() > index) {
            char addLetter = word.charAt(index);
            if (node.next.get(addLetter) == null) {
                node.next.put(addLetter, new Node());
            }
            add(node.next.get(addLetter), word, index + 1);
        }
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        Node cur = root;
        return contains(root, word, 0);
    }

    private boolean contains(Node node, String word, int index) {
        if (index == word.length()) {
            return node.isWord;
        }
        char c = word.charAt(index);
        if (node.next.get(c) == null) {
            return false;
        } else {
            return contains(node.next.get(c), word, index + 1);
        }
    }

    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        Node node = root;
        return recursionIsPrefix(root, prefix, 0);
    }

    public boolean recursionIsPrefix(Node root, String prefix, int index) {
        if(prefix.length() == index) {
            return true;
        }
        char c = prefix.charAt(index);
        if(root.next.get(c) == null) {
            return false;
        } else {
            return recursionIsPrefix(root.next.get(c), prefix, ++index);
        }
    }

}

/** * Your Trie object will be instantiated and called as such: * Trie obj = new Trie(); * obj.insert(word); * boolean param_2 = obj.search(word); * boolean param_3 = obj.startsWith(prefix); */

迭代实现:

package Trie;

import java.util.TreeMap;

// leetcode 208. 实现 Trie (前缀树)
class Trie2 {

    private class Node {
        public boolean isWord;
        public TreeMap<Character, Node> next;

        public Node(boolean isWord) {
            this.isWord = isWord;
            next = new TreeMap<>();
        }

        public Node() {
            this(false);
        }
    }

    private Node root;


    /** Initialize your data structure here. */
    public Trie2() {
        root = new Node();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        Node cur = root;
        // 取字符串中的字符,在树上进行查询,无则添加,有则下一个
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            if (cur.next.get(c) == null) {
                cur.next.put(c, new Node());
            }
            cur = cur.next.get(c);
        }
        // 此时一个单词已经遍历完成了,如果这个结束节点没有标记为单词,就标记为单词
        if (!cur.isWord) {
            cur.isWord = true;
        }
    }


    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        Node cur = root;
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            if (cur.next.get(c) == null) {
                return false;
            } else {
                cur = cur.next.get(c);
            }
        }
        return cur.isWord;
    }



    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        Node cur = root;
        for (int i = 0; i < prefix.length(); i++) {
            char c = prefix.charAt(i);
            if(cur.next.get(c) == null) {
                return false;
            }
            cur = cur.next.get(c);
        }
        return true;
    }


}

/** * Your Trie object will be instantiated and called as such: * Trie obj = new Trie(); * obj.insert(word); * boolean param_2 = obj.search(word); * boolean param_3 = obj.startsWith(prefix); */