Add CrosswordSolver algorithm (#5626)

src/test/java/com/thealgorithms/backtracking/CrosswordSolverTest.java

@@ -0,0 +1,66 @@
+package com.thealgorithms.backtracking;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.Arrays;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class CrosswordSolverTest {
+
+    @Test
+    public void testValidPlacement() {
+        char[][] puzzle = {{' ', ' ', ' '}, {' ', ' ', ' '}, {' ', ' ', ' '}};
+        assertTrue(CrosswordSolver.isValid(puzzle, "cat", 0, 0, true));
+        assertTrue(CrosswordSolver.isValid(puzzle, "dog", 0, 0, false));
+        assertFalse(CrosswordSolver.isValid(puzzle, "cat", 1, 2, false));
+    }
+
+    @Test
+    public void testPlaceAndRemoveWord() {
+        char[][] puzzle = {{' ', ' ', ' '}, {' ', ' ', ' '}, {' ', ' ', ' '}};
+        CrosswordSolver.placeWord(puzzle, "cat", 0, 0, true);
+        assertEquals('c', puzzle[0][0]);
+        assertEquals('a', puzzle[1][0]);
+        assertEquals('t', puzzle[2][0]);
+
+        CrosswordSolver.removeWord(puzzle, "cat", 0, 0, true);
+        assertEquals(' ', puzzle[0][0]);
+        assertEquals(' ', puzzle[1][0]);
+        assertEquals(' ', puzzle[2][0]);
+    }
+
+    @Test
+    public void testSolveCrossword() {
+        char[][] puzzle = {{' ', ' ', ' '}, {' ', ' ', ' '}, {' ', ' ', ' '}};
+        List<String> words = Arrays.asList("cat", "dog", "car");
+        assertTrue(CrosswordSolver.solveCrossword(puzzle, words));
+
+        /* Solved crossword:
+         * c d c
+         * a o a
+         * t g r
+         */
+
+        assertEquals('c', puzzle[0][0]);
+        assertEquals('a', puzzle[1][0]);
+        assertEquals('t', puzzle[2][0]);
+
+        assertEquals('d', puzzle[0][1]);
+        assertEquals('o', puzzle[1][1]);
+        assertEquals('g', puzzle[2][1]);
+
+        assertEquals('c', puzzle[0][2]);
+        assertEquals('a', puzzle[1][2]);
+        assertEquals('r', puzzle[2][2]);
+    }
+
+    @Test
+    public void testNoSolution() {
+        char[][] puzzle = {{' ', ' ', ' '}, {' ', ' ', ' '}, {' ', ' ', ' '}};
+        List<String> words = Arrays.asList("cat", "dog", "elephant"); // 'elephant' is too long for the grid
+        assertFalse(CrosswordSolver.solveCrossword(puzzle, words));
+    }
+}