LeetCode 实现Trie(C++)

题目:

实现一个 Trie (前缀树)，包含 insert, search, 和 startsWith 这三个操作。

示例:

Trie trie = new Trie();

trie.insert(“apple”);
trie.search(“apple”); // 返回 true
trie.search(“app”); // 返回 false
trie.startsWith(“app”); // 返回 true
trie.insert(“app”);
trie.search(“app”); // 返回 true

说明:

你可以假设所有的输入都是由小写字母 a-z 构成的。
保证所有输入均为非空字符串。

解题思路

这道题如果直接用vector来处理的话，将放入的数据放入一个vector<string>中自然是一种思路，具体代码如下:

class Trie {
public:
    /** Initialize your data structure here. */
    Trie() {
        vector<string> data(0);
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        this->data.push_back(word);
        return;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        for (auto x : this->data)
        {
            if (x == word)
            {
                return true;
            }
        }
        return false;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) 
    {
        int len = prefix.size();
        for (auto x : this->data)
        {
            if (x.substr(0, len) == prefix)
                return true;
        }
        return false;
    }
private:
    vector<string> data;
};

/**
 * 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);
 */

但是提交上述代码之后会出现超出时间限制的情况，因此我们需要改进一下这个数据结构，利用树结构来构造即可，将每个单词都当做一个节点，具体代码如下:

struct TrieNode
{
    public:
        bool flag;
        TrieNode* childnode[26] = {nullptr};
        TrieNode() : flag( false ) {};
};


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

/**
 * 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);
 */