JavaAlgorithms/Misc/WordBoggle.java

import java.util.*;

public class WordBoggle {

    /**
     * O(nm * 8^s + ws) time where n=width of boggle board, m=height of boggle board, s=length of
     * longest word in string array, w= length of string array, 8 is due to 8 explorable neighbours
     * O(nm + ws) space.
     */
    public static List<String> boggleBoard(char[][] board, String[] words) {
        Trie trie = new Trie();
        for (String word : words) trie.add(word);
        Set<String> finalWords = new HashSet<>();
        boolean[][] visited = new boolean[board.length][board.length];
        for (int i = 0; i < board.length; i++)
            for (int j = 0; j < board[i].length; j++)
                explore(i, j, board, trie.root, visited, finalWords);
        return new ArrayList<>(finalWords);
    }

    public static void main(String[] args) {
        // Testcase
        List<String> ans =
                new ArrayList<>(
                        Arrays.asList("a", "boggle", "this", "NOTRE_PEATED", "is", "simple", "board"));
        assert (boggleBoard(
                new char[][] {
                        {'t', 'h', 'i', 's', 'i', 's', 'a'},
                        {'s', 'i', 'm', 'p', 'l', 'e', 'x'},
                        {'b', 'x', 'x', 'x', 'x', 'e', 'b'},
                        {'x', 'o', 'g', 'g', 'l', 'x', 'o'},
                        {'x', 'x', 'x', 'D', 'T', 'r', 'a'},
                        {'R', 'E', 'P', 'E', 'A', 'd', 'x'},
                        {'x', 'x', 'x', 'x', 'x', 'x', 'x'},
                        {'N', 'O', 'T', 'R', 'E', '_', 'P'},
                        {'x', 'x', 'D', 'E', 'T', 'A', 'E'},
                },
                new String[] {
                        "this",
                        "is",
                        "not",
                        "a",
                        "simple",
                        "test",
                        "boggle",
                        "board",
                        "REPEATED",
                        "NOTRE_PEATED",
                })
                .equals(ans));
    }

    public static void explore(
            int i,
            int j,
            char[][] board,
            TrieNode trieNode,
            boolean[][] visited,
            Set<String> finalWords) {
        if (visited[i][j]) return;

        char letter = board[i][j];
        if (!trieNode.children.containsKey(letter)) {
            return;
        }
        visited[i][j] = true;
        trieNode = trieNode.children.get(letter);
        if (trieNode.children.containsKey('*')) finalWords.add(trieNode.word);

        List<Integer[]> neighbors = getNeighbors(i, j, board);
        for (Integer[] neighbor : neighbors)
            explore(neighbor[0], neighbor[1], board, trieNode, visited, finalWords);

        visited[i][j] = false;
    }

    public static List<Integer[]> getNeighbors(int i, int j, char[][] board) {
        List<Integer[]> neighbors = new ArrayList<>();
        if (i > 0 && j > 0) neighbors.add(new Integer[] {i - 1, j - 1});

        if (i > 0 && j < board[0].length - 1) neighbors.add(new Integer[] {i - 1, j + 1});

        if (i < board.length - 1 && j < board[0].length - 1)
            neighbors.add(new Integer[] {i + 1, j + 1});

        if (i < board.length - 1 && j > 0) neighbors.add(new Integer[] {i + 1, j - 1});

        if (i > 0) neighbors.add(new Integer[] {i - 1, j});

        if (i < board.length - 1) neighbors.add(new Integer[] {i + 1, j});

        if (j > 0) neighbors.add(new Integer[] {i, j - 1});

        if (j < board[0].length - 1) neighbors.add(new Integer[] {i, j + 1});

        return neighbors;
    }
}

// Trie used to optimize string search
class TrieNode {

    Map<Character, TrieNode> children = new HashMap<>();
    String word = "";
}

class Trie {

    TrieNode root;
    char endSymbol;

    public Trie() {
        this.root = new TrieNode();
        this.endSymbol = '*';
    }

    public void add(String str) {
        TrieNode node = this.root;
        for (int i = 0; i < str.length(); i++) {
            char letter = str.charAt(i);
            if (!node.children.containsKey(letter)) {
                TrieNode newNode = new TrieNode();
                node.children.put(letter, newNode);
            }
            node = node.children.get(letter);
        }
        node.children.put(this.endSymbol, null);
        node.word = str;
    }
}