LeetCode: 208. Implement Trie (Prefix Tree)

题目描述

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.

解题思路 —— 字典树

根据字典树的定义编码。

AC 代码

class Trie {
private:
    const static int CHILD_NODE_NUM = 27;
    struct TrieNode
    {
        TrieNode* ch[CHILD_NODE_NUM];
        size_t count;
        
        TrieNode():count(0)
        {
            for(size_t i = 0; i < CHILD_NODE_NUM; ++i)
            {
                ch[i] = nullptr;
            }
        }
    };
public:
    /** Initialize your data structure here. */
    Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        insert(root, word, 0);
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        return search(root, word, 0);
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        return startsWith(root, prefix, 0);
    }
private:
    void insert(TrieNode* r, string word, int idx)
    {
        int chIdx = CHILD_NODE_NUM-1;
      
        if(word.size() < idx)
        {
            return;
        }
        else if(word.size() != idx)
        {
            chIdx = word[idx]-'a';
        }
        
        if(r->ch[chIdx] == nullptr)
        {
            r->ch[chIdx] = new TrieNode();
        }
        
        ++(r->ch[chIdx]->count);
        
        insert(r->ch[chIdx], word, idx+1);
    }
    
    bool search(TrieNode* r, string word, int idx)
    {
        int chIdx = CHILD_NODE_NUM-1;
      
        if(word.size() < idx)
        {
            return r->count;
        }
        else if(word.size() != idx)
        {
            chIdx = word[idx]-'a';
        }
        
        if(r->ch[chIdx] == nullptr)
        {
            return false;
        }
        
        return search(r->ch[chIdx], word, idx+1);
    }
    bool startsWith(TrieNode* r, string word, int idx)
    {
        if(word.size() == idx)
        {
            return r->count;
        }
        
        if(r->ch[word[idx]-'a'] == nullptr)
        {
            return false;
        }
        
        return startsWith(r->ch[word[idx]-'a'], word, idx+1);
    }
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); */