Stream of Characters (H)

題目

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:

StreamChecker streamChecker = new StreamChecker(["cd","f","kl"]); // init the dictionary.
streamChecker.query('a');          // return false
streamChecker.query('b');          // return false
streamChecker.query('c');          // return false
streamChecker.query('d');          // return true, because 'cd' is in the wordlist
streamChecker.query('e');          // return false
streamChecker.query('f');          // return true, because 'f' is in the wordlist
streamChecker.query('g');          // return false
streamChecker.query('h');          // return false
streamChecker.query('i');          // return false
streamChecker.query('j');          // return false
streamChecker.query('k');          // return false
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.

題意

給定一個單詞字典，并按順序一連串字母，判斷字典中是否存在以當前輸入字母為結尾的單詞，

思路

找單詞一般可以用字典樹，同時注意到這里需要從單詞末尾找到單詞開頭，所以構建字典樹時也要倒著來，

代碼實作

Java

class StreamChecker {
    private Node root;
    private List<Character> queries;

    public StreamChecker(String[] words) {
        root = new Node();
        queries = new ArrayList<>();
        for (String word : words) {
            buildTrie(word);
        }
    }

    public boolean query(char letter) {
        queries.add(letter);
        Node p = root;
        for (int i = queries.size() - 1; i >= 0; i--) {
            char c = queries.get(i);
            if (p.children[c - 'a'] != null) {
                p = p.children[c - 'a'];
                if (p.end) {
                    return true;
                }
            } else {
                return false;
            }
        }
        return false;
    }

    private void buildTrie(String word) {
        Node p = root;
        for (int i = word.length() - 1; i >= 0; i--) {
            char c = word.charAt(i);
            if (p.children[c - 'a'] == null) {
                p.children[c - 'a'] = new Node();
            }
            p = p.children[c - 'a'];
        }
        p.end = true;
    }
}

class Node {
    Node[] children;
    boolean end;

    Node() {
        children = new Node[26];
        end = false;
    }
}

/**
 * Your StreamChecker object will be instantiated and called as such:
 * StreamChecker obj = new StreamChecker(words); boolean param_1 =
 * obj.query(letter);
 */

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1032. Stream of Characters (H)

Stream of Characters (H)

題目

題意

思路

代碼實作

Java