题目描述

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.

You should be familiar with how a Trie works. If not, please work on this problem: Implement Trie (Prefix Tree) first.

分析

本题只能用Trie，但是……LeetCode最近加入了一个测试用例，我试了所有网上能找到的代码，都是Time Limit Exceeded！

代码

    class TrieNode {

        boolean isLeaf;
        char c;
        HashMap<Character, TrieNode> children = new HashMap<Character, TrieNode>();

        // Initialize your data structure here.
        public TrieNode() {
        }

        public TrieNode(char c) {
            this.c = c;
        }
    }

    public class WordDictionary {

        private TrieNode root;

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

        // Adds a word into the data structure.
        public void addWord(String word) {

            HashMap<Character, TrieNode> children = root.children;

            for (int i = 0; i < word.length(); i++) {
                char c = word.charAt(i);
                TrieNode t;

                if (children.containsKey(c)) {
                    t = children.get(c);
                } else {
                    t = new TrieNode(c);
                    children.put(c, t);
                }

                children = t.children;
                if (i == word.length() - 1) {
                    t.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 searchNode(word, root);
        }

        private boolean searchNode(String word, TrieNode t) {

            if (t == null) {
                return false;
            }

            if (word.length() == 0) {
                return t.isLeaf;
            }

            HashMap<Character, TrieNode> children = t.children;
            char c = word.charAt(0);

            if (c == '.') {
                for (char key : children.keySet()) {
                    if (word.length() == 1 && children.get(key).isLeaf) {
                        return true;
                    }
                    if (searchNode(word.substring(1), children.get(key))) {
                        return true;
                    }
                }
                return false;
            } else if (!children.containsKey(c)) {
                return false;
            } else {
                return searchNode(word.substring(1), children.get(c));
            }

        }
    }

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