[EXERCISE] Trie traversing (https://en.wikipedia.org/wiki/Trie)

Trie.java

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

//[mama, maman, abama]
//
//a b a m
//b m a a
//z k n m
//b d q q
class Field {
    public static void main(String[] args) {
        Node trie = createTrie(Arrays.asList("mama", "maman", "obama"));

        char[][] field = {
                {'a', 'b', 'a', 'm'},
                {'b', 'm', 'a', 'a'},
                {'z', 'k', 'n', 'm'},
                {'b', 'd', 'q', 'q'}
        };

        for (int i = 0; i < field.length; ++i) {
            for (int j = 0; j < field[0].length; ++j) {
                if (hasWord(new boolean[field.length][field[0].length], field, i, j, trie)) {
                    System.out.println("Found word start: " + i + " " + j);
                }
            }
        }
    }

    private static boolean hasWord(boolean[][] prohibited, char[][] field, int i, int j, Node root) {
        throw new UnsupportedOperationException("Not finished yet");
    }

    private static Node createTrie(List<String> words) {
        Node root = new Node();
        fillNode(root, words, -1);
        return root;
    }

    private static void fillNode(Node node, List<String> words, int idx) {
        if (idx == -1) {//words does not have the same prefix
            Map<Character, List<String>> map = words.stream().collect(Collectors.groupingBy(w -> w.charAt(0), Collectors.toList()));
            for (Map.Entry<Character, List<String>> entry : map.entrySet()) {
                Node curr = new Node();
                curr.letter = entry.getKey();
                node.next.put(entry.getKey(), curr);
                fillNode(curr, entry.getValue(), 1);
            }

        } else {
            for (String w : words) {
                if (w.length() - 1 < idx) {
                    node.isTerminal = true;
                }
            }

            Map<Character, List<String>> map = words.stream().filter((w) -> w.length() - 1 >= idx).collect(Collectors.groupingBy(w -> w.charAt(idx), Collectors.toList()));
            for (Map.Entry<Character, List<String>> entry : map.entrySet()) {
                Node curr = new Node();
                curr.letter = entry.getKey();
                node.next.put(entry.getKey(), curr);
                fillNode(curr, entry.getValue(), idx + 1);
            }
        }
    }
}

class Node {
    public char letter = 0;
    public boolean isTerminal = false;
    public Map<Character, Node> next = new HashMap<>();
}