【leetcode】208. Implement Trie (Prefix Tree)

題目：

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

思路：

笨辦法，容器使用的unordered_set，搜尋時逐元素搜尋，效率相當相當低，

Runtime: 1080 ms

，不忍直視。

後來我發現，這是一個很重要的資料結構-字典樹（如下圖）。

我直接學習的https://www.cnblogs.com/grandyang/p/4491665.html，自己是真想不出來。思路就是設計一個多叉樹，每個節點儲存一個字元。關鍵是樹的節點結構，明白它就全明白了，如下：

class TrieNode{
public:
    TrieNode *child[26];
    TrieNode():isWord(false){
        for (auto &a : child){
            a = nullptr;
        }
    }
    bool isWord; // 如果此字元是單詞的最後一個字元，則說明到此為止是一個完整的單詞。
};
           

Runtime: 100 ms，運作時間降低了一個量級。

代碼實作：

蝸牛代碼：

class Trie {
public:
    /** Initialize your data structure here. */
    unordered_set<string> my_set;
    Trie() {
        
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        my_set.insert(word);
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        for (auto iter = my_set.begin(); iter != my_set.end(); ++iter){
            if (*iter == word){
                return true;
            }
        }
        return false;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        for (auto iter = my_set.begin(); iter != my_set.end(); ++iter){
            if (iter->find(prefix) == 0){
                return true;
            }
        }
        return false;
    }
};

/**
 * 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);
 */
           

更高效的代碼：

class TrieNode{
public:
    TrieNode *child[26];
    TrieNode():isWord(false){
        for (auto &a : child){
            a = nullptr;
        }
    }
    bool isWord;
};

class Trie {
public:

    /** Initialize your data structure here. */
    Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        TrieNode *p = root;
        for (auto &c : word){
            int i = c - 'a';
            if (p->child[i] == nullptr){
                p->child[i] = new TrieNode();
            }
            p = p->child[i];
        }
        p->isWord = true;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        TrieNode *p = root;
        for(auto &c : word){
            int i = c - 'a';
            if (p->child[i] == nullptr){
                return false;
            }
            p = p->child[i];
        }
        
        return p->isWord;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        TrieNode *p = root;
        for(auto &c : prefix){
            int i = c - 'a';
            if (p->child[i] == nullptr){
                return false;
            }
            p = p->child[i];
        }
        return true;
    }
private:
    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);
 */
           

參考：

https://www.cnblogs.com/grandyang/p/4491665.html

http://dongxicheng.org/structure/trietree/的圖