208 Implement Trie (Prefix Tree)

Implement a trie with insert, search, and startsWith methods.

Solution

class TrieNode {
public:
    char val;
    // NOTE THIS!!!
    bool isEnd;
    map<char, TrieNode*> children;
    // Initialize your data structure here.
    TrieNode(): isEnd(false){
    }
    TrieNode(char c): val(c), isEnd(false) {}
};

class Trie {
public:
    Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    void insert(string word) {
        TrieNode* p = root;
        int n = word.size();
        for ( int i = 0; i <= n-1; i++ ) {
            char c = word[i];
            if ( p->children.find(c) != p->children.end() ) p = p->children[c];
            else {
                TrieNode *node = new TrieNode(c);
                p->children[c] = node;
                p = p->children[c];
            }
        }
        p->isEnd = true;
        return;
    }

    // Returns if the word is in the trie.
    bool search(string word) {
        TrieNode* p = lastP(word);
        if ( p == NULL ) return false;
        return p->isEnd;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    bool startsWith(string prefix) {
        TrieNode* p = lastP(prefix);
        return ( p != NULL );
    }

private:
    TrieNode* root;
    TrieNode* lastP(string s) {
        TrieNode* p = root;
        int n = s.size();
        for ( int i = 0; i <= n-1; i++ ) {
            char c = s[i];
            if ( p->children.find(c) == p->children.end() ) {return NULL;}
            else p = p->children[c];
        }
        return p;
    }
};

// Your Trie object will be instantiated and called as such:
// Trie trie;
// trie.insert("somestring");
// trie.search("key");