Add CrosswordSolver.java new algorithm

DIRECTORY.md

@@ -10,6 +10,7 @@
             * [AllPathsFromSourceToTarget](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/backtracking/AllPathsFromSourceToTarget.java)
             * [ArrayCombination](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/backtracking/ArrayCombination.java)
             * [Combination](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/backtracking/Combination.java)
+            * [CrosswordSolver](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/backtracking/CrosswordSolver.java)
             * [FloodFill](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/backtracking/FloodFill.java)
             * [KnightsTour](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/backtracking/KnightsTour.java)
             * [MazeRecursion](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/backtracking/MazeRecursion.java)
@@ -616,6 +617,7 @@
             * [AllPathsFromSourceToTargetTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/backtracking/AllPathsFromSourceToTargetTest.java)
             * [ArrayCombinationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/backtracking/ArrayCombinationTest.java)
             * [CombinationTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/backtracking/CombinationTest.java)
+            * [CrosswordSolverTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/backtracking/CrosswordSolverTest.java)
             * [FloodFillTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/backtracking/FloodFillTest.java)
             * [KnightsTourTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/backtracking/KnightsTourTest.java)
             * [MazeRecursionTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/backtracking/MazeRecursionTest.java)

src/main/java/com/thealgorithms/backtracking/CrosswordSolver.java

@@ -0,0 +1,124 @@
+package com.thealgorithms.backtracking;
+
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * A class to solve a crossword puzzle using backtracking.
+ * Example:
+ * Input:
+ *  puzzle = {
+ *      {' ', ' ', ' '},
+ *      {' ', ' ', ' '},
+ *      {' ', ' ', ' '}
+ *  }
+ *  words = List.of("cat", "dog")
+ *
+ * Output:
+ *  {
+ *      {'c', 'a', 't'},
+ *      {' ', ' ', ' '},
+ *      {'d', 'o', 'g'}
+ *  }
+ */
+public final class CrosswordSolver {
+    private CrosswordSolver() {
+    }
+
+    /**
+     * Checks if a word can be placed at the specified position in the crossword.
+     *
+     * @param puzzle   The crossword puzzle represented as a 2D char array.
+     * @param word     The word to be placed.
+     * @param row      The row index where the word might be placed.
+     * @param col      The column index where the word might be placed.
+     * @param vertical If true, the word is placed vertically; otherwise, horizontally.
+     * @return true if the word can be placed, false otherwise.
+     */
+    public static boolean isValid(char[][] puzzle, String word, int row, int col, boolean vertical) {
+        for (int i = 0; i < word.length(); i++) {
+            if (vertical) {
+                if (row + i >= puzzle.length || puzzle[row + i][col] != ' ') {
+                    return false;
+                }
+            } else {
+                if (col + i >= puzzle[0].length || puzzle[row][col + i] != ' ') {
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+
+    /**
+     * Places a word at the specified position in the crossword.
+     *
+     * @param puzzle   The crossword puzzle represented as a 2D char array.
+     * @param word     The word to be placed.
+     * @param row      The row index where the word will be placed.
+     * @param col      The column index where the word will be placed.
+     * @param vertical If true, the word is placed vertically; otherwise, horizontally.
+     */
+    public static void placeWord(char[][] puzzle, String word, int row, int col, boolean vertical) {
+        for (int i = 0; i < word.length(); i++) {
+            if (vertical) {
+                puzzle[row + i][col] = word.charAt(i);
+            } else {
+                puzzle[row][col + i] = word.charAt(i);
+            }
+        }
+    }
+
+    /**
+     * Removes a word from the specified position in the crossword.
+     *
+     * @param puzzle   The crossword puzzle represented as a 2D char array.
+     * @param word     The word to be removed.
+     * @param row      The row index where the word is placed.
+     * @param col      The column index where the word is placed.
+     * @param vertical If true, the word was placed vertically; otherwise, horizontally.
+     */
+    public static void removeWord(char[][] puzzle, String word, int row, int col, boolean vertical) {
+        for (int i = 0; i < word.length(); i++) {
+            if (vertical) {
+                puzzle[row + i][col] = ' ';
+            } else {
+                puzzle[row][col + i] = ' ';
+            }
+        }
+    }
+
+    /**
+     * Solves the crossword puzzle using backtracking.
+     *
+     * @param puzzle The crossword puzzle represented as a 2D char array.
+     * @param words  The list of words to be placed.
+     * @return true if the crossword is solved, false otherwise.
+     */
+    public static boolean solveCrossword(char[][] puzzle, List<String> words) {
+        // Create a mutable copy of the words list
+        List<String> remainingWords = new ArrayList<>(words);
+
+        for (int row = 0; row < puzzle.length; row++) {
+            for (int col = 0; col < puzzle[0].length; col++) {
+                if (puzzle[row][col] == ' ') {
+                    for (String word : new ArrayList<>(remainingWords)) {
+                        for (boolean vertical : new boolean[] {true, false}) {
+                            if (isValid(puzzle, word, row, col, vertical)) {
+                                placeWord(puzzle, word, row, col, vertical);
+                                remainingWords.remove(word);
+                                if (solveCrossword(puzzle, remainingWords)) {
+                                    return true;
+                                }
+                                remainingWords.add(word);
+                                removeWord(puzzle, word, row, col, vertical);
+                            }
+                        }
+                    }
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+}

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));
+    }
+}