208. Implement Trie (Prefix Tree)

• Difficulty: Medium

• Topics: Design, Trie

• Similar Questions:

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Problem:

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.

Solutions:

class Trie {
public:
    struct TrieNode{
        bool isWord;
        TrieNode* next[26];
        TrieNode(bool isWord = false) {
            this->isWord = isWord;
            for (int i = 0; i < 26; ++i) {
                next[i] = nullptr;
            }
        }
    };

    /** Initialize your data structure here. */
    Trie() {
        root = new TrieNode(false);
    }

    /** Inserts a word into the trie. */
    void insert(string word) {
        TrieNode* lastNode = findNode(word, true);
        lastNode->isWord = true;
    }

    /** Returns if the word is in the trie. */
    bool search(string word) {
        TrieNode* lastNode = findNode(word, false);
        return lastNode != nullptr && lastNode->isWord;
    }

    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        return findNode(prefix, false) != nullptr;
    }
private:
    TrieNode* findNode(string& word, bool modifiable) {
        TrieNode* cur = root;
        for (auto& c : word) {
            if (cur->next[c - 'a'] == nullptr) {
                if (!modifiable)    return nullptr;
                cur->next[c -'a'] = new TrieNode(false);
            }
            cur = cur->next[c - 'a'];
        }
        return cur;
    }

    TrieNode* root;  
};

/**
 * 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);
 */