diff --git a/src/main/java/com/thealgorithms/puzzlesandgames/TowerOfHanoi.java b/src/main/java/com/thealgorithms/puzzlesandgames/TowerOfHanoi.java index 72e9a14ac070..d94bef69cd3a 100644 --- a/src/main/java/com/thealgorithms/puzzlesandgames/TowerOfHanoi.java +++ b/src/main/java/com/thealgorithms/puzzlesandgames/TowerOfHanoi.java @@ -3,27 +3,32 @@ import java.util.List; /** - * The {@code TowerOfHanoi} class provides a recursive solution to the Tower of Hanoi puzzle. - * This puzzle involves moving a set of discs from one pole to another, following specific rules: + * Recursive solution to the Tower of Hanoi puzzle. + * + *

+ * The puzzle rules are: * 1. Only one disc can be moved at a time. * 2. A disc can only be placed on top of a larger disc. * 3. All discs must start on one pole and end on another. + *

* - * This implementation recursively calculates the steps required to solve the puzzle and stores them - * in a provided list. + *

+ * The recursion follows three steps: + * 1. Move {@code n-1} discs from start to intermediate. + * 2. Move the largest disc from start to end. + * 3. Move {@code n-1} discs from intermediate to end. + *

* *

- * For more information about the Tower of Hanoi, see - * Tower of Hanoi on Wikipedia. + * Time Complexity: O(2^n) - exponential due to recursive expansion. + * Space Complexity: O(n) - recursion stack depth. *

* - * The {@code shift} method takes the number of discs and the names of the poles, - * and appends the steps required to solve the puzzle to the provided list. - * Time Complexity: O(2^n) - Exponential time complexity due to the recursive nature of the problem. - * Space Complexity: O(n) - Linear space complexity due to the recursion stack. - * Wikipedia: https://en.wikipedia.org/wiki/Tower_of_Hanoi + *

+ * See Tower of Hanoi on Wikipedia. + *

*/ -final class TowerOfHanoi { +public final class TowerOfHanoi { private TowerOfHanoi() { } @@ -36,6 +41,7 @@ private TowerOfHanoi() { * @param intermediatePole The name of the intermediate pole used as a temporary holding area. * @param endPole The name of the end pole to which discs are moved. * @param result A list to store the steps required to solve the puzzle. + * @throws IllegalArgumentException if {@code n} is negative. * *

* This method is called recursively to move n-1 discs @@ -51,15 +57,20 @@ private TowerOfHanoi() { *

*/ public static void shift(int n, String startPole, String intermediatePole, String endPole, List result) { - if (n != 0) { - // Move n-1 discs from startPole to intermediatePole - shift(n - 1, startPole, endPole, intermediatePole, result); + if (n < 0) { + throw new IllegalArgumentException("Number of discs must be non-negative"); + } + if (n == 0) { + return; + } - // Add the move of the nth disc from startPole to endPole - result.add(String.format("Move %d from %s to %s", n, startPole, endPole)); + // Move n-1 discs from startPole to intermediatePole + shift(n - 1, startPole, endPole, intermediatePole, result); - // Move the n-1 discs from intermediatePole to endPole - shift(n - 1, intermediatePole, startPole, endPole, result); - } + // Add the move of the nth disc from startPole to endPole + result.add(String.format("Move %d from %s to %s", n, startPole, endPole)); + + // Move the n-1 discs from intermediatePole to endPole + shift(n - 1, intermediatePole, startPole, endPole, result); } } diff --git a/src/test/java/com/thealgorithms/puzzlesandgames/TowerOfHanoiTest.java b/src/test/java/com/thealgorithms/puzzlesandgames/TowerOfHanoiTest.java index 42669eb03bb4..f0a2686d3e4b 100644 --- a/src/test/java/com/thealgorithms/puzzlesandgames/TowerOfHanoiTest.java +++ b/src/test/java/com/thealgorithms/puzzlesandgames/TowerOfHanoiTest.java @@ -1,14 +1,31 @@ package com.thealgorithms.puzzlesandgames; 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.ArrayList; import java.util.List; +import java.util.stream.Stream; import org.junit.jupiter.api.Test; +import org.junit.jupiter.params.ParameterizedTest; +import org.junit.jupiter.params.provider.Arguments; +import org.junit.jupiter.params.provider.MethodSource; public class TowerOfHanoiTest { + @ParameterizedTest + @MethodSource("diskCountAndMoveCount") + void testMoveCountMatchesFormula(int disks, int expectedMoves) { + List result = new ArrayList<>(); + TowerOfHanoi.shift(disks, "A", "B", "C", result); + assertEquals(expectedMoves, result.size()); + } + + private static Stream diskCountAndMoveCount() { + return Stream.of(Arguments.of(1, 1), Arguments.of(2, 3), Arguments.of(3, 7), Arguments.of(4, 15), Arguments.of(5, 31), Arguments.of(10, 1023)); + } + @Test public void testHanoiWithOneDisc() { List result = new ArrayList<>(); @@ -39,6 +56,15 @@ public void testHanoiWithThreeDiscs() { assertEquals(expected, result); } + @Test + public void testHanoiWithDifferentPoles() { + List result = new ArrayList<>(); + TowerOfHanoi.shift(2, "X", "Y", "Z", result); + + List expected = List.of("Move 1 from X to Y", "Move 2 from X to Z", "Move 1 from Y to Z"); + assertEquals(expected, result); + } + @Test public void testHanoiWithZeroDiscs() { List result = new ArrayList<>(); @@ -47,4 +73,10 @@ public void testHanoiWithZeroDiscs() { // There should be no moves if there are 0 discs assertTrue(result.isEmpty()); } + + @Test + public void testHanoiWithNegativeDiscsThrows() { + List result = new ArrayList<>(); + assertThrows(IllegalArgumentException.class, () -> TowerOfHanoi.shift(-1, "Pole1", "Pole2", "Pole3", result)); + } }