[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.

解法：

　　Trie（字典树）的知识参见：数据结构之Trie树 和 [LeetCode] Implement Trie (Prefix Tree) 实现字典树(前缀树)。

　　可以采用数组和哈希表的方式实现，代码分别如下：

public class Trie {
    private TrieNode root;

    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        root.insert(word, 0);
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode result = root.search(word, 0);
        return result != null && result.getIsWord();
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode result = root.search(prefix, 0);
        return result != null;
    }
}

class TrieNode {
    private TrieNode[] children;
    private boolean isWord;
    
    public TrieNode() {
        children = new TrieNode[26];
        isWord = false;
    }
    
    public void insert(String word, int index) {
        // 如果所有字符都已插入，需要将最后一个字符节点的isWord改为true
        if (index == word.length()) {
            this.isWord = true;
            return;
        }
        // 如果不存在该字符，在对应位置新建节点
        int n = word.charAt(index) - 'a';
        if (children[n] == null) {
            children[n] = new TrieNode();
        }
        // 继续下一字符
        children[n].insert(word, index + 1);
    }
    
    // 由于Trie中既要求实现search，又要求实现startsWith，为了方便，
    // 此处直接返回搜索结果的TrieNode，交由Trie去判断。
    public TrieNode search(String word, int index) {
        if (index == word.length()) {
            return this;
        }
        int n = word.charAt(index) - 'a';
        if (children[n] == null) {
            return null;
        }
        return children[n].search(word, index + 1);
    }
    
    public boolean getIsWord() {
        return this.isWord;
    }
}

/**
 * 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);
 */

public class Trie {
    private TrieNode root;

    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode curr = root;
        for (int i = 0; i < word.length(); i++) {
            char letter = word.charAt(i);
            if (!curr.children.containsKey(letter)) {
                curr.children.put(letter, new TrieNode());
            }
            curr = curr.children.get(letter);
        }
        curr.isWord = true;
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode result = find(word);
        return result != null && result.isWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode result = find(prefix);
        return result != null;
    }
    
    public TrieNode find(String word) {
        TrieNode curr = root;
        for (int i = 0; i < word.length(); i++) {
            char letter = word.charAt(i);
            if (!curr.children.containsKey(letter)) {
                return null;
            }
            curr = curr.children.get(letter);
        }
        return curr;
    }
}

class TrieNode {
    HashMap<Character, TrieNode> children;
    boolean isWord;
    
    public TrieNode() {
        children = new HashMap<>();
        isWord = false;
    }
}

/**
 * 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);
 */