LeetCode 208. Implement Trie (Prefix Tree)

Trie树主要用于字符串前缀匹配，树的根节点不存元素，其余节点存一个字符。对于一个字符串，其实就是顺着一条路径搜索的过程。

TrieNode节点有一个bool表示是否是一个word的结束，还有一个TrieNode *的数组，指向所有的孩子。

插入和搜索的时间复杂度都是 O(n)，n表示给定字符串的长度。

使用 Trie Tree，也可以非常简单找到所有前缀匹配上的字符串。

class TrieNode{
public:
    bool isend;     // if the end of word, isend=true
    vector<TrieNode *> next;
    TrieNode():isend(false), next(26,NULL){}
};


class Trie {
public:
    /** Initialize your data structure here. */
    Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    void insert(string word) {
        if (search(word)) return;
        TrieNode *p=root;
        for (char ch:word){
            if (p->next[ch-'a']==NULL)
                p->next[ch-'a'] = new TrieNode();
            p = p->next[ch-'a'];
        }
        p->isend = true;
    }
    
    /** Returns if the word is in the trie. */
    bool search(string word) {
        TrieNode *p=root;
        for (char ch:word){
            p = p->next[ch-'a'];
            if (p==NULL) return false;
        }
        return p->isend==true;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    bool startsWith(string prefix) {
        TrieNode *p=root;
        for (char ch:prefix){
            p = p->next[ch-'a'];
            if (p==NULL) return false;
        }
        dfs(p,prefix);
        return true;
    }
    
    void dfs(TrieNode *root, string s){
        if (root->isend) cout<<s<<endl;
        for (char ch='a';ch<='z';++ch){
            if (root->next[ch-'a'])
                dfs(root->next[ch-'a'],s+ch);
        }
    }

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);
 */

References:

https://blog.csdn.net/hackbuteer1/article/details/7964147

https://leetcode.com/problems/implement-trie-prefix-tree/discuss/58842/Maybe-the-code-is-not-too-much-by-using-%22next26%22-C++