diff --git a/src/main/java/com/thealgorithms/backtracking/RatInAMaze.java b/src/main/java/com/thealgorithms/backtracking/RatInAMaze.java new file mode 100644 index 000000000000..183b4bbd97f8 --- /dev/null +++ b/src/main/java/com/thealgorithms/backtracking/RatInAMaze.java @@ -0,0 +1,119 @@ +package com.thealgorithms.backtracking; + +import java.util.ArrayList; +import java.util.List; + +/** + * Rat in a Maze Problem using Backtracking. + * + *

Given an {@code n x n} binary maze where {@code 1} represents an open cell + * and {@code 0} represents a blocked cell, find all paths for a rat starting at + * the top-left cell {@code (0, 0)} to reach the bottom-right cell {@code (n-1, n-1)}. + * + *

The rat can move in four directions: Up (U), Down (D), Left (L), Right (R). + * Each cell may be visited at most once per path. + * + *

Time Complexity: O(4^(n²)) in the worst case (four choices per cell). + * Space Complexity: O(n²) for the visited matrix and recursion stack. + * + *

Example: + *

+ *   maze = { {1, 0, 0, 0},
+ *            {1, 1, 0, 1},
+ *            {0, 1, 0, 0},
+ *            {0, 1, 1, 1} }
+ *   Output: ["DDRDRR", "DRDDRR"]  (two valid paths)
+ * 
+ * + * @see Maze solving algorithm + * @author the-Sunny-Sharma (GitHub) + */ +public final class RatInAMaze { + + private RatInAMaze() { + } + + /** + * Finds all paths from the top-left to the bottom-right of the given maze. + * + * @param maze an {@code n x n} binary matrix where {@code 1} = open, {@code 0} = blocked + * @return a sorted list of all valid path strings using directions D, L, R, U; + * an empty list if no path exists + * @throws IllegalArgumentException if the maze is null, empty, or not square + */ + public static List findPaths(final int[][] maze) { + if (maze == null || maze.length == 0) { + throw new IllegalArgumentException("Maze must not be null or empty."); + } + int n = maze.length; + for (int[] row : maze) { + if (row.length != n) { + throw new IllegalArgumentException("Maze must be a square (n x n) matrix."); + } + } + List results = new ArrayList<>(); + if (maze[0][0] == 0 || maze[n - 1][n - 1] == 0) { + return results; + } + boolean[][] visited = new boolean[n][n]; + solve(maze, 0, 0, n, "", visited, results); + return results; + } + + /** + * Recursive backtracking helper that explores all four directions. + * + * @param maze the binary maze + * @param row current row position + * @param col current column position + * @param n maze dimension + * @param path path string built so far + * @param visited tracks visited cells for the current path + * @param results accumulates complete paths + */ + private static void solve(final int[][] maze, final int row, final int col, final int n, final String path, final boolean[][] visited, final List results) { + // Base case: reached destination + if (row == n - 1 && col == n - 1) { + results.add(path); + return; + } + + // Mark current cell as visited + visited[row][col] = true; + + // Explore in alphabetical order: Down, Left, Right, Up + // Down + if (isSafe(maze, row + 1, col, n, visited)) { + solve(maze, row + 1, col, n, path + 'D', visited, results); + } + // Left + if (isSafe(maze, row, col - 1, n, visited)) { + solve(maze, row, col - 1, n, path + 'L', visited, results); + } + // Right + if (isSafe(maze, row, col + 1, n, visited)) { + solve(maze, row, col + 1, n, path + 'R', visited, results); + } + // Up + if (isSafe(maze, row - 1, col, n, visited)) { + solve(maze, row - 1, col, n, path + 'U', visited, results); + } + + // Backtrack: unmark current cell + visited[row][col] = false; + } + + /** + * Checks whether moving to {@code (row, col)} is valid. + * + * @param maze the binary maze + * @param row target row + * @param col target column + * @param n maze dimension + * @param visited tracks visited cells for the current path + * @return {@code true} if the cell is within bounds, open, and not yet visited + */ + private static boolean isSafe(final int[][] maze, final int row, final int col, final int n, final boolean[][] visited) { + return row >= 0 && row < n && col >= 0 && col < n && maze[row][col] == 1 && !visited[row][col]; + } +} diff --git a/src/test/java/com/thealgorithms/backtracking/RatInAMazeTest.java b/src/test/java/com/thealgorithms/backtracking/RatInAMazeTest.java new file mode 100644 index 000000000000..ecd1f3c4dfae --- /dev/null +++ b/src/test/java/com/thealgorithms/backtracking/RatInAMazeTest.java @@ -0,0 +1,99 @@ +package com.thealgorithms.backtracking; + +import static org.junit.jupiter.api.Assertions.assertEquals; +import static org.junit.jupiter.api.Assertions.assertThrows; +import static org.junit.jupiter.api.Assertions.assertTrue; + +import java.util.List; +import org.junit.jupiter.api.Test; + +class RatInAMazeTest { + + @Test + void testMultiplePathsExist() { + int[][] maze = {{1, 0, 0, 0}, {1, 1, 0, 1}, {0, 1, 0, 0}, {0, 1, 1, 1}}; + + List paths = RatInAMaze.findPaths(maze); + assertTrue(paths.size() >= 1); + for (String path : paths) { + assertTrue(path.chars().allMatch(c -> "DLRU".indexOf(c) >= 0)); + } + } + + @Test + void testSinglePath() { + int[][] maze = {{1, 0, 0}, {1, 1, 0}, {0, 1, 1}}; + List paths = RatInAMaze.findPaths(maze); + assertEquals(1, paths.size()); + assertEquals("DRDR", paths.get(0)); + } + + @Test + void testNoPathExists() { + int[][] maze = {{1, 0, 0}, {0, 0, 0}, {0, 0, 1}}; + List paths = RatInAMaze.findPaths(maze); + assertTrue(paths.isEmpty()); + } + + @Test + void testSourceBlocked() { + int[][] maze = {{0, 1}, {1, 1}}; + List paths = RatInAMaze.findPaths(maze); + assertTrue(paths.isEmpty()); + } + + @Test + void testDestinationBlocked() { + int[][] maze = {{1, 1}, {1, 0}}; + List paths = RatInAMaze.findPaths(maze); + assertTrue(paths.isEmpty()); + } + + @Test + void testSingleCellMazeOpen() { + int[][] maze = {{1}}; + List paths = RatInAMaze.findPaths(maze); + assertEquals(1, paths.size()); + assertEquals("", paths.get(0)); + } + + @Test + void testSingleCellMazeBlocked() { + int[][] maze = {{0}}; + List paths = RatInAMaze.findPaths(maze); + assertTrue(paths.isEmpty()); + } + + @Test + void testNullMazeThrowsException() { + assertThrows(IllegalArgumentException.class, () -> RatInAMaze.findPaths(null)); + } + + @Test + void testEmptyMazeThrowsException() { + assertThrows(IllegalArgumentException.class, () -> RatInAMaze.findPaths(new int[][] {})); + } + + @Test + void testNonSquareMazeThrowsException() { + int[][] maze = {{1, 0, 1}, {1, 1, 1}}; + assertThrows(IllegalArgumentException.class, () -> RatInAMaze.findPaths(maze)); + } + + @Test + void testAllCellsOpen() { + int[][] maze = {{1, 1, 1}, {1, 1, 1}, {1, 1, 1}}; + List paths = RatInAMaze.findPaths(maze); + assertTrue(paths.size() > 1); + } + + @Test + void testLargerMazeWithPath() { + int[][] maze = {{1, 1, 1, 1}, {0, 1, 0, 1}, {0, 1, 0, 1}, {0, 1, 1, 1}}; + List paths = RatInAMaze.findPaths(maze); + assertTrue(paths.size() >= 1); + for (String path : paths) { + assertTrue(path.chars().allMatch(c -> "DLRU".indexOf(c) >= 0), "Path contains invalid characters: " + path); + } + } +}