1032. Stream of Characters - Hard

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.

 

Store the words in the trie with reverse order, and check the query string from the end

class StreamChecker {
    class TrieNode {
        TrieNode[] children = new TrieNode[26];
        boolean isWord;
    }

    TrieNode root;
    StringBuilder sb;

    public StreamChecker(String[] words) {
        root = new TrieNode();
        sb = new StringBuilder();
        buildTrie(words);
    }

    private void buildTrie(String[] words) {
        for(String w : words) {
            TrieNode cur = root;
            for(int i = w.length() - 1; i >= 0; i--) {
                if(cur.children[w.charAt(i) - 'a'] == null) {
                    cur.children[w.charAt(i) - 'a'] = new TrieNode();
                }
                cur = cur.children[w.charAt(i) - 'a'];
            }
            cur.isWord = true;
        }
    }

    public boolean query(char letter) {
        sb.append(letter);
        TrieNode cur = root;
        for(int i = sb.length() - 1; i >= 0 && cur != null; i--) {
            cur = cur.children[sb.charAt(i) - 'a'];
            if(cur != null && cur.isWord) {
                return true;
            }
        }
        return false;
    }
}

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