LeetCode208 Implement Trie (Prefix Tree). LeetCode211 Add and Search Word - Data structure design

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[];
     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[];
     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 = ; 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 = ; 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 = ; 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[];
     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 = ; 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 + , curRoot -> child[word[pos] - 'a']);
             }
         }
         else {
             bool flag = false;
             for (int i = ; i < ; ++i) {
                 if (curRoot -> child[i] != NULL && helper(word, pos + , curRoot -> child[i]) ) {
                     flag = true;
                 }
             }
             return flag;
         }
         return true;
     }
     // Returns if the word is in the trie.
     bool search(string word) {
         return helper(word, , 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");