diff --git a/src/main/java/com/thealgorithms/datastructures/queues/ReverseQueueRecursion.java b/src/main/java/com/thealgorithms/datastructures/queues/ReverseQueueRecursion.java new file mode 100644 index 000000000000..79275dcefe20 --- /dev/null +++ b/src/main/java/com/thealgorithms/datastructures/queues/ReverseQueueRecursion.java @@ -0,0 +1,28 @@ +package com.thealgorithms.datastructures.queues; + +import java.util.Queue; + +/** + * Reverse a queue using recursion. + */ +public final class ReverseQueueRecursion { + private ReverseQueueRecursion() { + // private constructor to prevent instantiation + } + + /** + * Reverses the given queue recursively. + * + * @param queue the queue to reverse + * @param the type of elements in the queue + */ + public static void reverseQueue(final Queue queue) { + if (queue == null || queue.isEmpty()) { + return; + } + + final T front = queue.poll(); + reverseQueue(queue); + queue.add(front); + } +} diff --git a/src/test/java/com/thealgorithms/datastructures/queues/ReverseQueueRecursionTest.java b/src/test/java/com/thealgorithms/datastructures/queues/ReverseQueueRecursionTest.java new file mode 100644 index 000000000000..e3abe15b6a46 --- /dev/null +++ b/src/test/java/com/thealgorithms/datastructures/queues/ReverseQueueRecursionTest.java @@ -0,0 +1,54 @@ +package com.thealgorithms.datastructures.queues; + +import static org.junit.jupiter.api.Assertions.assertEquals; +import static org.junit.jupiter.api.Assertions.assertTrue; + +import java.util.LinkedList; +import java.util.Queue; +import org.junit.jupiter.api.Test; + +class ReverseQueueRecursionTest { + @Test + void testReverseMultipleElements() { + Queue queue = new LinkedList<>(); + queue.add(1); + queue.add(2); + queue.add(3); + queue.add(4); + ReverseQueueRecursion.reverseQueue(queue); + assertEquals(4, queue.poll()); + assertEquals(3, queue.poll()); + assertEquals(2, queue.poll()); + assertEquals(1, queue.poll()); + assertTrue(queue.isEmpty()); + } + + @Test + void testReverseSingleElement() { + Queue queue = new LinkedList<>(); + queue.add(42); + ReverseQueueRecursion.reverseQueue(queue); + assertEquals(42, queue.poll()); + assertTrue(queue.isEmpty()); + } + + @Test + void testReverseEmptyQueue() { + Queue queue = new LinkedList<>(); + ReverseQueueRecursion.reverseQueue(queue); + assertTrue(queue.isEmpty()); + } + + @Test + void testReverseStringQueue() { + Queue queue = new LinkedList<>(); + queue.add("A"); + queue.add("B"); + queue.add("C"); + ReverseQueueRecursion.reverseQueue(queue); + assertEquals("C", queue.poll()); + assertEquals("B", queue.poll()); + assertEquals("A", queue.poll()); + assertTrue(queue.isEmpty()); + } +}