0208. 實現 Trie (前綴樹) · ttttt

# 0208. 實現 Trie (前綴樹) ## 題目地址(208. 實現 Trie (前綴樹)) <https://leetcode-cn.com/problems/implement-trie-prefix-tree/> ## 題目描述 ``` <pre class="calibre18">``` 實現一個 Trie (前綴樹)，包含 insert, search, 和 startsWith 這三個操作。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 說明: 你可以假設所有的輸入都是由小寫字母 a-z 構成的。保證所有輸入均為非空字符串。 ``` ``` ## 前置知識 - 前綴樹 ## 公司 - 阿里 - 騰訊 - 百度 - 字節 ## 思路這是一道很直接的題目，上來就讓你實現`前綴樹（字典樹）`。這算是基礎數據結構中的知識了，不清楚什么是字典樹的可以查閱相關資料。我們看到題目給出的使用方法`new Trie`, `insert`,`search`和`startWith`. 為了區分`search`和`startWith`我們需要增加一個標示來區分當前節點是否是某個單詞的結尾。因此節點的數據結構應該是: ``` <pre class="calibre18">``` function TrieNode(val) { this.val = val; // 當前的字母 this.children = []; // 題目要求字典僅有a-z，那么其長度最大為26（26個字母） this.isWord = false; } ``` ``` 每次 insert 我們其實都是從根節點出發，一個一個找到我們需要添加的節點，修改 children 的值. 我們應該修改哪一個 child 呢？我們需要一個函數來計算索引 ``` <pre class="calibre18">``` function computeIndex(c) { return c.charCodeAt(0) - "a".charCodeAt(0); } ``` ``` 其實不管 insert， search 和 startWith 的邏輯都是差不多的，都是從 root 出發，找到我們需要操作的 child，然后進行相應操作（添加，修改，返回）。 ![](https://img.kancloud.cn/08/e0/08e038e4c5e4db4840ad43b89c7ddde5_827x602.jpg) ## 關鍵點解析 - 前綴樹 ## 代碼 ``` <pre class="calibre18">``` /* * @lc app=leetcode id=208 lang=javascript * * [208] Implement Trie (Prefix Tree) * * https://leetcode.com/problems/implement-trie-prefix-tree/description/ * * algorithms * Medium (36.93%) * Total Accepted: 172K * Total Submissions: 455.5K * Testcase Example: '["Trie","insert","search","search","startsWith","insert","search"]\n[[],["apple"],["apple"],["app"],["app"],["app"],["app"]]' * * Implement a trie with insert, search, and startsWith methods. * * Example: * * * Trie trie = new Trie(); * * trie.insert("apple"); * trie.search("apple"); // returns true * trie.search("app"); // returns false * trie.startsWith("app"); // returns true * trie.insert("app"); * trie.search("app"); // returns true * * * Note: * * * You may assume that all inputs are consist of lowercase letters a-z. * All inputs are guaranteed to be non-empty strings. * * */ function TrieNode(val) { this.val = val; this.children = []; this.isWord = false; } function computeIndex(c) { return c.charCodeAt(0) - "a".charCodeAt(0); } /** * Initialize your data structure here. */ var Trie = function () { this.root = new TrieNode(null); }; /** * Inserts a word into the trie. * @param {string} word * @return {void} */ Trie.prototype.insert = function (word) { let ws = this.root; for (let i = 0; i < word.length; i++) { const c = word[i]; const current = computeIndex(c); if (!ws.children[current]) { ws.children[current] = new TrieNode(c); } ws = ws.children[current]; } ws.isWord = true; }; /** * Returns if the word is in the trie. * @param {string} word * @return {boolean} */ Trie.prototype.search = function (word) { let ws = this.root; for (let i = 0; i < word.length; i++) { const c = word[i]; const current = computeIndex(c); if (!ws.children[current]) return false; ws = ws.children[current]; } return ws.isWord; }; /** * Returns if there is any word in the trie that starts with the given prefix. * @param {string} prefix * @return {boolean} */ Trie.prototype.startsWith = function (prefix) { let ws = this.root; for (let i = 0; i < prefix.length; i++) { const c = prefix[i]; const current = computeIndex(c); if (!ws.children[current]) return false; ws = ws.children[current]; } return true; }; /** * Your Trie object will be instantiated and called as such: * var obj = new Trie() * obj.insert(word) * var param_2 = obj.search(word) * var param_3 = obj.startsWith(prefix) */ ``` ``` ## 相關題目 - [0211.add-and-search-word-data-structure-design](211.add-and-search-word-data-structure-design.html) - [0212.word-search-ii](212.word-search-ii.html) - [0472.concatenated-words](problems/472.concatenated-words.md) - [0820.short-encoding-of-words](https://github.com/azl397985856/leetcode/blob/master/problems/820.short-encoding-of-words.md) - [1032.stream-of-characters](https://github.com/azl397985856/leetcode/blob/master/problems/1032.stream-of-characters.md)