WordSearchII.java

import java.util.*;

class WordSearchII {
    private class TrieNode {
        TrieNode[] children = new TrieNode[26];
        String word = null;
    }
    
    private TrieNode root;

    public List<String> findWords(char[][] board, String[] words) {
        List<String> result = new ArrayList<>();
        if (board == null || board.length == 0 || board[0].length == 0 || words == null || words.length == 0) {
            return result;
        }

        // Build the Trie
        root = new TrieNode();
        for (String word : words) {
            insert(word);
        }

        // Start DFS from each cell
        int rows = board.length;
        int cols = board[0].length;
        for (int row = 0; row < rows; row++) {
            for (int col = 0; col < cols; col++) {
                if (root.children[board[row][col] - 'a'] != null) {
                    dfs(board, row, col, root, result);
                }
            }
        }

        return result;
    }

    private void insert(String word) {
        TrieNode node = root;
        for (char c : word.toCharArray()) {
            int index = c - 'a';
            if (node.children[index] == null) {
                node.children[index] = new TrieNode();
            }
            node = node.children[index];
        }
        node.word = word;
    }

    private void dfs(char[][] board, int row, int col, TrieNode node, List<String> result) {
        char c = board[row][col];
        if (c == '#' || node.children[c - 'a'] == null) {
            return;
        }

        node = node.children[c - 'a'];
        if (node.word != null) {
            result.add(node.word);
            node.word = null; // Avoid duplicate entries
        }

        // Mark the cell as visited
        board[row][col] = '#';
        int[][] directions = { {0, 1}, {1, 0}, {0, -1}, {-1, 0} };
        for (int[] direction : directions) {
            int newRow = row + direction[0];
            int newCol = col + direction[1];
            if (newRow >= 0 && newRow < board.length && newCol >= 0 && newCol < board[0].length) {
                dfs(board, newRow, newCol, node, result);
            }
        }
        // Restore the cell
        board[row][col] = c;
    }

    public static void main(String[] args) {
        WordSearchIIPart2 solution = new WordSearchIIPart2();
        char[][] board = {
            {'o','a','a','n'},
            {'e','t','a','e'},
            {'i','h','k','r'},
            {'i','f','l','v'}
        };
        String[] words = {"oath", "pea", "eat", "rain"};
        System.out.println(solution.findWords(board, words)); // Output: [oath, eat]
        
        
        char[][] board1 = {{'a','b'},{'c','d'}};
        String[] words1 = {"abcb", "bd"};
        System.out.println(solution.findWords(board1, words1)); // Output: []
        
    }
}