字典树

    字典树，又叫Trie树，也称前缀树。典型应用是用于统计、排序和保存大量的字符串，经常被搜索引擎系统用于文本词频统计。优点：利用公共前缀来减少查询时间，最大限度地减少查询比较的次数。

struct TrieNode{
    char c;
    bool isLeaf;
    TrieNode* children[26];
    TrieNode(){
        c=0;
        isLeaf=false;
        memset(children,NULL,sizeof(TrieNode*)*26);
    }
    TrieNode(char var){
        c=var;
        isLeaf=false;
        memset(children,NULL,sizeof(TrieNode*)*26);
    }
};

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) {
        if (word.empty())
            return;
        TrieNode* p=root;
        for (int i=0;i<word.size();i++){
            char var=word[i];
            if (!p->children[var-'a']){
                p->children[var-'a']=new TrieNode(word[i]);
            }
            p=p->children[var-'a'];
        }
        p->isLeaf=true;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        if (word.empty())
            return true;
        TrieNode* p=root;
        for (int i=0;i<word.size();i++){
            char var=word[i];
            if (p->children[var-'a']){
                p=p->children[var-'a'];
            }
            else
                return false;
        }
        if (p->isLeaf)
            return true;
        else
            return false;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        if (prefix.empty())
            return true;
        TrieNode* p=root;
        for (int i=0;i<prefix.size();i++){
            char var=prefix[i];
            if (p->children[var-'a']){
                p=p->children[var-'a'];
            }
            else
                return false;
        }
        return true;
    }
    
    void freeTrieNode(TrieNode* root){
        if (NULL==root)
            return;
        TrieNode* p=root;
        for (int i=0;i<26;i++){
            if (p->children[i])
                freeTrieNode(p->children[i]);
        }
        free(p);
    }
    
    ~Trie(){
        freeTrieNode(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);
 */