Add and Search Word - Data structure design 解答

Question

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.

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.

Solution

This kind of problem (searching a String) can be solved by Trie.

We use DFS to implement search. Notice corner cases.

When we choose to implement it by recursion, think one step each time.

 class TrieNode {
     public char value;
     public boolean isLeaf;
     public HashMap<Character, TrieNode> children;

     public TrieNode(char c) {
         value = c;
         children = new HashMap<Character, TrieNode>();
         isLeaf = false;
     }
 }

 public class WordDictionary {
     public TrieNode root;

     public WordDictionary() {
         root = new TrieNode('!');
     }

     // Adds a word into the data structure.
     public void addWord(String word) {
         TrieNode currentNode = root;
         for (int i = 0; i < word.length(); i++) {
             char tmp = word.charAt(i);
             HashMap<Character, TrieNode> children = currentNode.children;
             TrieNode nextNode;
             if (children.containsKey(tmp)) {
                 nextNode = children.get(tmp);
             } else {
                 nextNode = new TrieNode(tmp);
                 children.put(tmp, nextNode);
             }
             currentNode = nextNode;
             // Check whether it's the last character
             if (i == word.length() - 1)
                 currentNode.isLeaf = true;
         }
     }

     // Returns if the word is in the data structure. A word could
     // contain the dot character '.' to represent any one letter.
     public boolean search(String word) {
         return dfsSearch(word, 0, root);
     }

     private boolean dfsSearch(String word, int index, TrieNode prevNode) {
         // If prevNode is null but word has not been completely traversed, return fasle
         if (prevNode == null) {
             return false;
         }

         // If word has been completely traversed, check whether tree is at bottom
         if (index == word.length()) {
             return prevNode.isLeaf;
         }
         char target = word.charAt(index);
         HashMap<Character, TrieNode> currentMap = prevNode.children;

         if (target != '.') {
             if (!currentMap.containsKey(target))
                 return false;
             else
                 return dfsSearch(word, index + 1, currentMap.get(target));
         } else {
             boolean result = false;
             for (Character key : currentMap.keySet()) {
                 if (dfsSearch(word, index + 1, currentMap.get(key))) {
                     result = true;
                 }
             }
             return result;
         }
     }
 }

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