LeetCode-in-Cpp

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

#include <string>
#include <vector>

class Trie {
private:
    struct TrieNode {
        std::vector<TrieNode*> children;
        bool isWord;

        TrieNode() : children(26, nullptr), isWord(false) {}
    };

    TrieNode* root;
    bool startWith;

    void insert(const std::string& word, TrieNode* node, int idx) {
        if (idx == word.length()) {
            node->isWord = true;
            return;
        }
        int index = word[idx] - 'a';
        if (node->children[index] == nullptr) {
            node->children[index] = new TrieNode();
        }
        insert(word, node->children[index], idx + 1);
    }

    bool search(const std::string& word, TrieNode* node, int idx) {
        if (idx == word.length()) {
            startWith = true;
            return node->isWord;
        }
        int index = word[idx] - 'a';
        if (node->children[index] == nullptr) {
            startWith = false;
            return false;
        }
        return search(word, node->children[index], idx + 1);
    }

public:
    Trie() : root(new TrieNode()), startWith(false) {}

    void insert(const std::string& word) {
        insert(word, root, 0);
    }

    bool search(const std::string& word) {
        return search(word, root, 0);
    }

    bool startsWith(const std::string& prefix) {
        search(prefix, root, 0);
        return startWith;
    }
};

/**
 * Your Trie object will be instantiated and called as such:
 * Trie* obj = new Trie();
 * obj->insert(word);
 * bool param_2 = obj->search(word);
 * bool param_3 = obj->startsWith(prefix);
 */

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