208. Implement Trie (Prefix Tree)--Trie树

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本文介绍了一种使用C++实现Trie树（字典树）的方法，包括insert、search和startsWith等核心功能。采用栈上空间分配的方式管理节点，提供插入、搜索及前缀匹配操作。

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

Note:
You may assume that all inputs are consist of lowercase letters a-z.

我的解法如下：

const int MAX_NODE = 26;

struct TrieNode {
    long count;
    TrieNode* next[MAX_NODE];
    TrieNode() : count(0) { memset(next, 0, sizeof(next)); }
};

TrieNode memory[100000];
int allocp = 0;

class Trie {
public:
    /** Initialize your data structure here. */
    Trie() : root(create())
    {}

    ~Trie() {
        destroy(root);
    }

    /** Inserts a word into the trie. */
    void insert(string word) {
        TrieNode* tmp = root;
        for(int i=0; i<word.length(); ++i){
            int index = word[i] - 'a';
            if(tmp->next[index] == NULL)
                tmp->next[index] = create();
            tmp = tmp->next[index];
        }
        tmp->count++; //只有在成为一个单词后count++
    }

    /** Returns if the word is in the trie. */
    bool search(string word) {
        TrieNode* tmp = find(word);
        return tmp != NULL && tmp->count != 0;
    }

    /** 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* create() {
        return &memory[allocp++];
    }
    /* 不需要析构，因为采用了栈上空间memory
    void destroy(TrieNode* t) {
        for(int i=0; i<MAX_NODE; ++i){
            if(t->next[i] != NULL)
                destroy(t->next[i]);
        }
        delete t;
    }
    */
    TrieNode* find(string& key){
        TrieNode* tmp = root;
        for(int i=0; i<key.length() && tmp!=NULL; ++i)
            tmp = tmp->next[key[i]-'a'];
        return tmp;
    }
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);
 */