题目

Implement the StreamChecker class as follows:

  • StreamChecker(words): Constructor, init the data structure with the given words.
  • query(letter): returns true if and only if for some k >= 1, the last k characters queried (in order from oldest to newest, including this letter just queried) spell one of the words in the given list.

Example:

  1. StreamChecker streamChecker = new StreamChecker(["cd","f","kl"]); // init the dictionary.
  2. streamChecker.query('a');          // return false
  3. streamChecker.query('b');          // return false
  4. streamChecker.query('c');          // return false
  5. streamChecker.query('d');          // return true, because 'cd' is in the wordlist
  6. streamChecker.query('e');          // return false
  7. streamChecker.query('f');          // return true, because 'f' is in the wordlist
  8. streamChecker.query('g');          // return false
  9. streamChecker.query('h');          // return false
  10. streamChecker.query('i');          // return false
  11. streamChecker.query('j');          // return false
  12. streamChecker.query('k');          // return false
  13. streamChecker.query('l');          // return true, because 'kl' is in the wordlist

Note:

  • 1 <= words.length <= 2000
  • 1 <= words[i].length <= 2000
  • Words will only consist of lowercase English letters.
  • Queries will only consist of lowercase English letters.
  • The number of queries is at most 40000.

题意

给定一个单词字典，并按顺序一连串字母，判断字典中是否存在以当前输入字母为结尾的单词。

思路

找单词一般可以用字典树，同时注意到这里需要从单词末尾找到单词开头，所以构建字典树时也要倒着来。

代码实现

Java

  1. class StreamChecker {
  2.     private Node root;
  3.     private List<Character> queries;
  5.     public StreamChecker(String[] words) {
  6.         root = new Node();
  7.         queries = new ArrayList<>();
  8.         for (String word : words) {
  9.             buildTrie(word);
  10.         }
  11.     }
  13.     public boolean query(char letter) {
  14.         queries.add(letter);
  15.         Node p = root;
  16.         for (int i = queries.size() - 1; i >= 0; i--) {
  17.             char c = queries.get(i);
  18.             if (p.children[c - 'a'] != null) {
  19.                 p = p.children[c - 'a'];
  20.                 if (p.end) {
  21.                     return true;
  22.                 }
  23.             } else {
  24.                 return false;
  25.             }
  26.         }
  27.         return false;
  28.     }
  30.     private void buildTrie(String word) {
  31.         Node p = root;
  32.         for (int i = word.length() - 1; i >= 0; i--) {
  33.             char c = word.charAt(i);
  34.             if (p.children[c - 'a'] == null) {
  35.                 p.children[c - 'a'] = new Node();
  36.             }
  37.             p = p.children[c - 'a'];
  38.         }
  39.         p.end = true;
  40.     }
  41. }
  43. class Node {
  44.     Node[] children;
  45.     boolean end;
  47.     Node() {
  48.         children = new Node[26];
  49.         end = false;
  50.     }
  51. }
  53. /**
  54.  * Your StreamChecker object will be instantiated and called as such:
  55.  * StreamChecker obj = new StreamChecker(words); boolean param_1 =
  56.  * obj.query(letter);
  57.  */