LeetCode208 Implement Trie (Prefix Tree). LeetCode211 Add and Search Word

字典树（Trie树相关）

208. Implement Trie (Prefix Tree)

Implement a trie with insert, search, and startsWith methods. （Medium）

Note:
You may assume that all inputs are consist of lowercase letters a-z.

分析：

字典树即前缀匹配树，在空间不是很影响的情况下一般采用如下数据结构存储Trie节点

class TrieNode {
public:
    // Initialize your data structure here.
    bool isWord;
    TrieNode* child[26];
    TrieNode(): isWord(false){
        memset(child, NULL, sizeof(child));
    }
};

所以插入、查找等操作比较直观，直接见程序。

代码：

class TrieNode {
public:
    // Initialize your data structure here.
    bool isWord;
    TrieNode* child[26];
    TrieNode(): isWord(false){
        memset(child, NULL, sizeof(child));
    }
};

class Trie {
public:
    Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    void insert(string word) {
        TrieNode* r = root;
        for (int i = 0; i < word.size(); ++i) {
            if (r -> child[word[i] - 'a'] == NULL) {
                r -> child[word[i] - 'a'] = new TrieNode();

            }
            r = r -> child[word[i] - 'a'];
        }
        r -> isWord = true;
    }

    // Returns if the word is in the trie.
    bool search(string word) {
        TrieNode* r = root;
        for (int i = 0; i < word.size(); ++i) {
            if (r -> child[word[i] - 'a'] == NULL) {
                return false;

            }
            r = r -> child[word[i] - 'a'];
        }
        if (r -> isWord) {
            return true;
        }
        return false;
    }

    // Returns if there is any word in the trie
    // that starts with the given prefix.
    bool startsWith(string prefix) {
        TrieNode* r = root;
        for (int i = 0; i < prefix.size(); ++i) {
            if (r -> child[prefix[i] - 'a'] == NULL) {
                return false;

            }
            r = r -> child[prefix[i] - 'a'];
        }
        return true;
    }

private:
    TrieNode* root;
};

// Your Trie object will be instantiated and called as such:
// Trie trie;
// trie.insert("somestring");
// trie.search("key");

211. Add and Search Word - Data structure design

Design a data structure that supports the following two operations:

void addWord(word)
bool search(word)

search(word) can search a literal word or a regular expression string containing only letters a-z or .. A . means it can represent any one letter. （Medium）

For example:

addWord("bad")
addWord("dad")
addWord("mad")
search("pad") -> false
search("bad") -> true
search(".ad") -> true
search("b..") -> true

Note:
You may assume that all words are consist of lowercase letters a-z.

分析：

继续沿用字典树的思路，只不过加入了“.”通配符，所以在查找过程中加入DFS即可，用辅助函数helper

代码：

class TrieNode {
public:
    // Initialize your data structure here.
    bool isWord;
    TrieNode* child[26];
    TrieNode(): isWord(false){
        memset(child, NULL, sizeof(child));
    }
};

class Trie {
public:
    Trie() {
        root = new TrieNode();
    }

    // Inserts a word into the trie.
    void insert(string word) {
        TrieNode* r = root;
        for (int i = 0; i < word.size(); ++i) {
            if (r -> child[word[i] - 'a'] == NULL) {
                r -> child[word[i] - 'a'] = new TrieNode();

            }
            r = r -> child[word[i] - 'a'];
        }
        r -> isWord = true;
    }

    //helper function for search, use to solve "." problem 
    bool helper(string word, int pos, TrieNode* curRoot) {
        if (pos == word.size() && curRoot -> isWord) {
            return true;
        }
        if (word[pos] != '.') {
            if (curRoot -> child[word[pos] - 'a'] == NULL) {
                return false;
            }
            else {
                return helper(word, pos + 1, curRoot -> child[word[pos] - 'a']);
            }
        }
        else {
            bool flag = false;
            for (int i = 0; i < 26; ++i) {
                if (curRoot -> child[i] != NULL && helper(word, pos + 1, curRoot -> child[i]) ) {
                    flag = true;
                }
            }
            return flag;
        }
        return true;
    }
    // Returns if the word is in the trie.
    bool search(string word) {
        return helper(word, 0, root);
    }

private:
    TrieNode* root;
};

class WordDictionary {
public:
    Trie dic;
    // Adds a word into the data structure.
    void addWord(string word) {
        dic.insert(word);
    }

    // Returns if the word is in the data structure. A word could
    // contain the dot character '.' to represent any one letter.
    bool search(string word) {
        return dic.search(word);
    }
};

// Your WordDictionary object will be instantiated and called as such:
// WordDictionary wordDictionary;
// wordDictionary.addWord("word");
// wordDictionary.search("pattern");