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208. Implement Trie (Prefix Tree)

Medium

A trie (pronounced as “try”) or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker.

Implement the Trie class:

• Trie() Initializes the trie object.
• void insert(String word) Inserts the string word into the trie.
• boolean search(String word) Returns true if the string word is in the trie (i.e., was inserted before), and false otherwise.
• boolean startsWith(String prefix) Returns true if there is a previously inserted string word that has the prefix prefix, and false otherwise.

Example 1:

Input

["Trie", "insert", "search", "search", "startsWith", "insert", "search"]
[[], ["apple"], ["apple"], ["app"], ["app"], ["app"], ["app"]]

Output: [null, null, true, false, true, null, true]

Explanation:

Trie trie = new Trie();
trie.insert("apple"); trie.search("apple"); // return True
trie.search("app"); // return False
trie.startsWith("app"); // return True
trie.insert("app");
trie.search("app"); // return True 

Constraints:

• 1 <= word.length, prefix.length <= 2000
• word and prefix consist only of lowercase English letters.
• At most 3 * 104 calls in total will be made to insert, search, and startsWith.

Solution

class TrieNode {
    children: TrieNode[]
    isWord: boolean

    constructor() {
        this.children = new Array<TrieNode>(26)
        this.isWord = false
    }
}

class Trie {
    private root: TrieNode
    private startWith: boolean

    private static readonly ALPHABET_SIZE = 26

    constructor() {
        this.root = new TrieNode()
        this.startWith = false
    }

    // Inserts a word into the trie.
    insert(word: string): void {
        this.insertRecursive(word, this.root, 0)
    }

    private insertRecursive(word: string, node: TrieNode, idx: number): void {
        if (idx === word.length) {
            node.isWord = true
            return
        }
        const charIndex = this.getCharIndex(word.charAt(idx))
        if (!node.children[charIndex]) {
            node.children[charIndex] = new TrieNode()
        }
        this.insertRecursive(word, node.children[charIndex], idx + 1)
    }

    // Returns if the word is in the trie.
    search(word: string): boolean {
        this.startWith = false
        return this.searchRecursive(word, this.root, 0)
    }

    private searchRecursive(word: string, node: TrieNode, idx: number): boolean {
        if (idx === word.length) {
            this.startWith = true
            return node.isWord
        }

        const charIndex = this.getCharIndex(word.charAt(idx))
        if (!node.children[charIndex]) {
            this.startWith = false
            return false
        }
        return this.searchRecursive(word, node.children[charIndex], idx + 1)
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    startsWith(prefix: string): boolean {
        this.search(prefix)
        return this.startWith
    }

    private getCharIndex(char: string): number {
        return char.charCodeAt(0) - 'a'.charCodeAt(0)
    }
}

/*
 * 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)
 */

export { Trie }

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