[100天算法】-实现 Trie（day 41）

题目描述

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。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/implement-trie-prefix-tree
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

思路

熟悉一下 Trie 的概念就可以了。

https://medium.com/basecs/trying-to-understand-tries-3ec6bede0014

复杂度分析

时间复杂度： $O(L)$ ，L 是字符串长度， insert search startsWith 操作都是。
空间复杂度： $O(M\^{L})$ ，L 是字符串长度，M 是字符集中字符个数，如本题中 M 就是 26。

代码

TypeScript Code

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class TrieNode {
    value: string;
    children: Array<TrieNode | null>;

    constructor(value) {
        this.value = value;
        this.children = Array(26);
    }
}

class Trie {
    private root: TrieNode;

    constructor() {
        this.root = this._getTrieNode('');
    }

    private _getTrieNode(value: string): TrieNode {
        return new TrieNode(value);
    }

    private _char2Index(char: string): number {
        return char.toLowerCase().charCodeAt(0) - 97;
    }

    insert(word: string): void {
        let crawl: TrieNode = this.root;
        for (let char of word) {
            const index: number = this._char2Index(char);
            if (!crawl.children[index]) {
                crawl.children[index] = this._getTrieNode('');
            }
            crawl = crawl.children[index];
        }
        crawl.value = word;
    }

    search(word: string): boolean {
        let crawl: TrieNode = this.root;
        for (let char of word) {
            const index: number = this._char2Index(char);
            if (!crawl.children[index]) return false;
            crawl = crawl.children[index];
        }
        return crawl.value === word;
    }

    startsWith(prefix: string): boolean {
        let crawl: TrieNode = this.root;
        for (let char of prefix) {
            const index: number = this._char2Index(char);
            if (!crawl.children[index]) return false;
            crawl = crawl.children[index];
        }
        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)
 */

JavaScript Code

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class TrieNode {
    constructor(val) {
        this.value = val;
        this.pointers = Array(26);
    }
}

class Trie {
    constructor() {
        this.root = this._getTrieNode('');
    }

    /**
     * @param {string} val
     */
    _getTrieNode(val) {
        return new TrieNode(val);
    }

    /**
     * @param {string} char
     * @returns {number}
     */
    _char2Index(char) {
        return char.toLowerCase().charCodeAt(0) - 97;
    }

    /**
     * Inserts a word into the trie.
     * @param {string} word
     * @return {void}
     */
    insert(word) {
        let crawl = this.root;
        for (let char of word) {
            const index = this._char2Index(char);
            if (!crawl.pointers[index]) {
                crawl.pointers[index] = this._getTrieNode('');
            }
            crawl = crawl.pointers[index];
        }
        // Store the word in the last TrieNode as an end mark.
        crawl.value = word;
    }

    /**
     * Returns if the word is in the trie.
     * @param {string} word
     * @return {boolean}
     */
    search(word) {
        let crawl = this.root;
        for (let char of word) {
            const index = this._char2Index(char);
            if (!crawl.pointers[index]) return false;

            crawl = crawl.pointers[index];
        }
        // If it has a stored value, it is the last TrieNode, i.e., the desired word is found.
        // Otherwise, the word doesn't exist in Trie.
        return !!crawl.value;
    }

    /**
     * Returns if there is any word in the trie that starts with the given prefix.
     * @param {string} prefix
     * @return {boolean}
     */
    startsWith(prefix) {
        let crawl = this.root;
        for (let char of prefix) {
            const index = this._char2Index(char);
            if (!crawl.pointers[index]) return false;

            crawl = crawl.pointers[index];
        }
        return true;
    }
}

输入输出

Nodejs

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const __main__ = function () {
    const readline = require('readline');
    const rl = readline.createInterface({
        input: process.stdin,
        output: process.stdout,
    });

    console.log('******输入******');
    rl.prompt();
    const lines = [];
    rl.on('line', line => lines.push(line));

    rl.on('close', () => {
        console.log('\n******输出******');

        const test = (operations, params) => {
            if (operations[0] === 'Trie') {
                const trie = new Trie();
                const output = [null];
                for (let i = 1; i < operations.length; i++) {
                    const res = trie[operations[i]](...params[i]);
                    output.push(res === void 0 ? null : res);
                }
                console.log(output);
            } else {
                console.log(Array(operations.length).fill(null));
            }
        };

        while (lines.length >= 2) {
            const params = lines.splice(0, 2);
            test(...params.map(el => JSON.parse(el)));
        }
    });
};