> For the complete documentation index, see [llms.txt](https://mayanktyagi3111.gitbook.io/interview-prep/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mayanktyagi3111.gitbook.io/interview-prep/trees/stream-of-characters.md).

# Stream of Characters

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.

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

class StreamChecker {

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

    TrieNode root = new TrieNode();
    StringBuilder sb = new StringBuilder();

    public StreamChecker(String[] words) {
        createTrie(words);
    }

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

    private void createTrie(String[] words) {
        for (String s : words) {
            TrieNode node = root;
            int len = s.length();
            for (int i = len - 1; i >= 0; i--) {
                char c = s.charAt(i);
                if (node.next[c - 'a'] == null) {
                    node.next[c - 'a'] = new TrieNode();
                }
                node = node.next[c - 'a'];
            }
            node.isWord = true;
        }
    }
}
```