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| 1 | +### 描述: |
| 2 | +实现一个 Trie (前缀树),包含 insert, search, 和 startsWith 这三个操作。 |
| 3 | + |
| 4 | +### 示例: |
| 5 | +``` |
| 6 | +Trie trie = new Trie(); |
| 7 | + |
| 8 | +trie.insert("apple"); |
| 9 | +trie.search("apple"); // 返回 true |
| 10 | +trie.search("app"); // 返回 false |
| 11 | +trie.startsWith("app"); // 返回 true |
| 12 | +trie.insert("app"); |
| 13 | +trie.search("app"); // 返回 true |
| 14 | +``` |
| 15 | + |
| 16 | +### 解析: |
| 17 | +定义TrieNode构造函数,其中nodes为该节点能通向的下一个字符节点,标注每个节点是否为结束节点。Trie构造函数中初始化root,insert构建整个trie树,search根据深度遍历来查找匹配路径。 |
| 18 | + |
| 19 | +单操作时间复杂度: O(n) |
| 20 | + |
| 21 | +```javascript |
| 22 | +var TrieNode = function() { |
| 23 | + this.nodes = {} |
| 24 | + this.isEnd = false |
| 25 | + |
| 26 | + this.setNode = function(char, node) { |
| 27 | + if(this.nodes[char]) return this.nodes[char] |
| 28 | + else { |
| 29 | + this.nodes[char] = node |
| 30 | + return node |
| 31 | + } |
| 32 | + } |
| 33 | + |
| 34 | + this.findNode = function(char) { |
| 35 | + return this.nodes[char] |
| 36 | + } |
| 37 | + |
| 38 | + this.setEnd = function() { |
| 39 | + this.isEnd = true |
| 40 | + } |
| 41 | +} |
| 42 | + |
| 43 | +/** |
| 44 | + * Initialize your data structure here. |
| 45 | + */ |
| 46 | +var Trie = function() { |
| 47 | + this.root = new TrieNode() |
| 48 | +}; |
| 49 | + |
| 50 | +/** |
| 51 | + * Inserts a word into the trie. |
| 52 | + * @param {string} word |
| 53 | + * @return {void} |
| 54 | + */ |
| 55 | +Trie.prototype.insert = function(word) { |
| 56 | + let cursor = this.root |
| 57 | + for(let i=0; i<word.length; i++) { |
| 58 | + const temp = new TrieNode() |
| 59 | + const node = cursor.setNode(word[i], temp) |
| 60 | + cursor = node |
| 61 | + } |
| 62 | + cursor.setEnd() |
| 63 | +}; |
| 64 | + |
| 65 | +/** |
| 66 | + * Returns if the word is in the trie. |
| 67 | + * @param {string} word |
| 68 | + * @return {boolean} |
| 69 | + */ |
| 70 | +Trie.prototype.search = function(word) { |
| 71 | + let cursor = this.root |
| 72 | + for(let i=0; i<word.length; i++) { |
| 73 | + const node = cursor.findNode(word[i]) |
| 74 | + if(!node) return false |
| 75 | + cursor = node |
| 76 | + } |
| 77 | + return cursor.isEnd |
| 78 | +}; |
| 79 | + |
| 80 | +/** |
| 81 | + * Returns if there is any word in the trie that starts with the given prefix. |
| 82 | + * @param {string} prefix |
| 83 | + * @return {boolean} |
| 84 | + */ |
| 85 | +Trie.prototype.startsWith = function(prefix) { |
| 86 | + let cursor = this.root |
| 87 | + for(let i=0; i<prefix.length; i++) { |
| 88 | + const node = cursor.findNode(prefix[i]) |
| 89 | + if(!node) return false |
| 90 | + cursor = node |
| 91 | + } |
| 92 | + return true |
| 93 | +}; |
| 94 | +``` |
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