208. Implement Trie (Prefix Tree)

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.

class Trie {
public:
    class TrieNode {
    public:
        vector<TrieNode*> next;
        bool isWord;
        TrieNode() : next(vector<TrieNode*>(26, nullptr)), isWord(false) {}
        ~TrieNode() {
            for (TrieNode*& ptr : next) {
                if (ptr) delete ptr;
            }
        }
    };
    /** Initialize your data structure here. */
    Trie() {
        root = new TrieNode();
    }
    ~Trie() {
        delete root;
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        TrieNode* cur = root;
        for (char ch : word) {
            if (!cur->next[ch - 'a']) {
                cur->next[ch - 'a'] = new TrieNode();
            }
            cur = cur->next[ch - 'a'];
        }
        cur->isWord = true;
    }
    
    TrieNode* find(string target) {
        TrieNode* cur = root;
        for (char ch : target) {
            if (!cur->next[ch - 'a']) return nullptr;
            cur = cur->next[ch - 'a'];
        }
        return cur;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        TrieNode* cur = find(word);
        return cur && cur->isWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        return find(prefix) != nullptr;
    }
private:
    TrieNode* root;
};