208. Implement Trie (Prefix Tree)
实现思路是每个结点保存一个26个指针数组，在插入的时候建立前后关系，查询完整单词依赖is_word，而查询前缀只要单词长度查完，没有出现NULL即匹配，完整代码如下(包含析构函数):

class TrieNode {
public:
    // Initialize your data structure here.
    bool is_word;
    TrieNode *children[26];
    
    TrieNode() {
        is_word = false;
        
        for (int i = 0; i < 26; i++)
            children[i] = NULL;
    }
};

class Trie {
public:
    /** Initialize your data structure here. */
    Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        int len = word.length();
        TrieNode * cur = root;
        for(int i = 0;i < len;i++){
            int k = word[i] - 'a';
            if(cur->children[k] == NULL){
                cur->children[k] = new TrieNode();
            }
            cur = cur->children[k];
        }
        //设置完整单词标志
        cur->is_word = true;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        TrieNode * cur = root;
        int len = word.length();
        for(int i = 0;i < len ;i++){
            int k = word[i] - 'a';
            if(cur->children[k] == NULL)
                return false;
            cur = cur->children[k];
        }
        
        if(cur->is_word){
            return true;
        }
        
        return false;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        TrieNode * cur = root;
        int len = prefix.length();
        for(int i = 0;i < len ;i++){
            int k = prefix[i] - 'a';
            if(cur->children[k] == NULL)
                return false;
            cur = cur->children[k];
        }
        
        return true;
    }
    
    ~Trie() {
        clear(root);
    }
private:
    TrieNode * root;
    void clear(TrieNode * cur)
    {
        for(int i = 0;i < 26;i++)
        {
            if(cur->children[i]){
                clear(cur->children[i]);
            }
        }
        delete cur;
    }
        
};

/**
 * 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);
 */

运行结果:

Runtime: 84 ms, faster than 67.60% of C++ online submissions for Implement Trie (Prefix Tree).
Memory Usage: 45.4 MB, less than 40.73% of C++ online submissions for Implement Trie (Prefix Tree).