208 Implement Trie (Prefix Tree)

1 题目

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.

2 尝试解

class Trie {
public:
    vector<string> saver;
    /** Initialize your data structure here. */
    Trie() {
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        saver.push_back(word);
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        return count(saver.begin(),saver.end(),word)!=0;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        for(string word:saver){
            if(word.size() < prefix.size()) continue;
            int flag = 1;
            for(int i = 0; i < prefix.size(); i++){
                if(word[i]!=prefix[i]){
                    flag = 0;
                    break;
                }
            }
            if(flag) return true;
        }
        return false;
    }
};

3 标准解

class TrieNode
{
public:
    TrieNode *next[26];
    bool is_word;
    
    // Initialize your data structure here.
    TrieNode(bool b = false)
    {
        memset(next, 0, sizeof(next));
        is_word = b;
    }
};

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

    // Inserts a word into the trie.
    void insert(string s)
    {
        TrieNode *p = root;
        for(int i = 0; i < s.size(); ++ i)
        {
            if(p -> next[s[i] - 'a'] == NULL)
                p -> next[s[i] - 'a'] = new TrieNode();
            p = p -> next[s[i] - 'a'];
        }
        p -> is_word = true;
    }

    // Returns if the word is in the trie.
    bool search(string key)
    {
        TrieNode *p = find(key);
        return p != NULL && p -> is_word;
    }

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

private:
    TrieNode* find(string key)
    {
        TrieNode *p = root;
        for(int i = 0; i < key.size() && p != NULL; ++ i)
            p = p -> next[key[i] - 'a'];
        return p;
    }
};