Add unit tests for the "Word Search" problem.

src/main/java/com/leetcode/medium/WordSearch79.java

@@ -0,0 +1,37 @@
+// Tags: Backtracking, DFS
+package com.leetcode.medium;
+
+public class WordSearch79 {
+    private boolean dfs(int r, int c, char[][] board, String word, int index, boolean[][] visited) {
+        if (index == word.length()) {
+            return true;
+        }
+        if (r < 0 || r == board.length || c < 0 || c == board[0].length || visited[r][c] || word.charAt(index) != board[r][c]) {
+            return false;
+        }
+        visited[r][c] = true;
+        if (this.dfs(r + 1, c, board, word, index + 1, visited) || this.dfs(r - 1, c, board, word, index + 1, visited) || this.dfs(r, c + 1, board, word, index + 1, visited) || this.dfs(r, c - 1, board, word, index + 1, visited)) {
+            return true;
+        }
+        visited[r][c] = false;
+        return false;
+    }
+    /*
+        w: length of word
+        Time complexity: O(m*n*4^w)
+        Space complexity: O(w)
+     */
+    public boolean exist(char[][] board, String word) {
+        int rows = board.length;
+        int cols = board[0].length;
+        boolean[][] visited = new boolean[rows][cols];
+        for (int r = 0; r < board.length; r++) {
+            for (int c = 0; c < board[0].length; c++) {
+                if (this.dfs(r, c, board, word, 0, visited)) {
+                    return true;
+                }
+            }
+        }
+        return false;
+    }
+}

src/test/java/com/leetcode/medium/WordSearch79Test.java

@@ -0,0 +1,51 @@
+package com.leetcode.medium;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import java.time.Duration;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTimeoutPreemptively;
+
+public class WordSearch79Test {
+    private WordSearch79 solution;
+
+    @BeforeEach
+    void setUp() {
+        solution = new WordSearch79();
+    }
+
+    @Test
+    void testExist_Example1() {
+        char[][] board = {{'A', 'B', 'C', 'E'}, {'S', 'F', 'C', 'S'}, {'A', 'D', 'E', 'E'}};
+        String word = "ABCCED";
+        boolean expected = true;
+
+        boolean result = assertTimeoutPreemptively(Duration.ofMillis(100), () -> solution.exist(board, word));
+
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void testExist_Example2() {
+        char[][] board = {{'A', 'B', 'C', 'E'}, {'S', 'F', 'C', 'S'}, {'A', 'D', 'E', 'E'}};
+        String word = "SEE";
+        boolean expected = true;
+
+        boolean result = assertTimeoutPreemptively(Duration.ofMillis(100), () -> solution.exist(board, word));
+
+        assertEquals(expected, result);
+    }
+
+    @Test
+    void testExist_Example3() {
+        char[][] board = {{'A', 'B', 'C', 'E'}, {'S', 'F', 'C', 'S'}, {'A', 'D', 'E', 'E'}};
+        String word = "ABCB";
+        boolean expected = false;
+
+        boolean result = assertTimeoutPreemptively(Duration.ofMillis(100), () -> solution.exist(board, word));
+
+        assertEquals(expected, result);
+    }
+}