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Enhance docs, remove main, add tests in `MergeSortedSingl… (#5997)
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‎DIRECTORY.md

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* lists
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* [CircleLinkedListTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/CircleLinkedListTest.java)
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* [CreateAndDetectLoopTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/CreateAndDetectLoopTest.java)
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* [MergeSortedSinglyLinkedListTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/MergeSortedSinglyLinkedListTest.java)
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* [QuickSortLinkedListTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/QuickSortLinkedListTest.java)
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* [ReverseKGroupTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/ReverseKGroupTest.java)
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* [RotateSinglyLinkedListsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/datastructures/lists/RotateSinglyLinkedListsTest.java)

‎src/main/java/com/thealgorithms/datastructures/lists/MergeSortedSinglyLinkedList.java

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package com.thealgorithms.datastructures.lists;
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/**
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* Utility class for merging two sorted singly linked lists.
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*
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* <p>This class extends the {@link SinglyLinkedList} class to support the merging of two sorted linked lists.
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* It provides a static method, `merge`, that takes two sorted singly linked lists, merges them into a single sorted linked list,
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* and returns the result.</p>
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*
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* <p>Example usage:</p>
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* <pre>
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* SinglyLinkedList listA = new SinglyLinkedList();
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* SinglyLinkedList listB = new SinglyLinkedList();
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* for (int i = 2; i <= 10; i += 2) {
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* listA.insert(i); // listA: 2->4->6->8->10
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* listB.insert(i - 1); // listB: 1->3->5->7->9
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* }
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* SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
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* System.out.println(mergedList); // Output: 1->2->3->4->5->6->7->8->9->10
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* </pre>
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*
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* <p>The `merge` method assumes that both input lists are already sorted in ascending order.
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* It returns a new singly linked list that contains all elements from both lists in sorted order.</p>
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*
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* @see SinglyLinkedList
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*/
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public class MergeSortedSinglyLinkedList extends SinglyLinkedList {
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public static void main(String[] args) {
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SinglyLinkedList listA = new SinglyLinkedList();
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SinglyLinkedList listB = new SinglyLinkedList();
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for (int i = 2; i <= 10; i += 2) {
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listA.insert(i);
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listB.insert(i - 1);
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}
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assert listA.toString().equals("2->4->6->8->10");
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assert listB.toString().equals("1->3->5->7->9");
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assert merge(listA, listB).toString().equals("1->2->3->4->5->6->7->8->9->10");
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}
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/**
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* Merge two sorted SingleLinkedList
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* Merges two sorted singly linked lists into a single sorted singly linked list.
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*
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* <p>This method does not modify the input lists; instead, it creates a new merged linked list
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* containing all elements from both lists in sorted order.</p>
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*
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* @param listA the first sorted list
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* @param listB the second sored list
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* @return merged sorted list
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* @param listA The first sorted singly linked list.
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* @param listB The second sorted singly linked list.
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* @return A new singly linked list containing all elements from both lists in sorted order.
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* @throws NullPointerException if either input list is null.
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*/
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public static SinglyLinkedList merge(SinglyLinkedList listA, SinglyLinkedList listB) {
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if (listA == null || listB == null) {
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throw new NullPointerException("Input lists must not be null.");
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}
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Node headA = listA.getHead();
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Node headB = listB.getHead();
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int size = listA.size() + listB.size();
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Node head = new Node();
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}
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tail = tail.next;
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}
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if (headA == null) {
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tail.next = headB;
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}
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if (headB == null) {
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tail.next = headA;
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}
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// Attach remaining nodes
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tail.next = (headA == null) ? headB : headA;
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return new SinglyLinkedList(head.next, size);
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}
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}

‎src/test/java/com/thealgorithms/datastructures/lists/MergeSortedSinglyLinkedListTest.java

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package com.thealgorithms.datastructures.lists;
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import static org.junit.jupiter.api.Assertions.assertEquals;
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import static org.junit.jupiter.api.Assertions.assertThrows;
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import org.junit.jupiter.api.Test;
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class MergeSortedSinglyLinkedListTest {
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@Test
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void testMergeTwoSortedLists() {
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SinglyLinkedList listA = new SinglyLinkedList();
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SinglyLinkedList listB = new SinglyLinkedList();
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for (int i = 2; i <= 10; i += 2) {
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listA.insert(i);
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listB.insert(i - 1);
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}
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SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
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assertEquals("1->2->3->4->5->6->7->8->9->10", mergedList.toString(), "Merged list should contain all elements in sorted order.");
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}
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@Test
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void testMergeWithEmptyListA() {
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SinglyLinkedList listA = new SinglyLinkedList(); // Empty listA
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SinglyLinkedList listB = new SinglyLinkedList();
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listB.insert(1);
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listB.insert(3);
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listB.insert(5);
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SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
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assertEquals("1->3->5", mergedList.toString(), "Merged list should match listB when listA is empty.");
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}
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@Test
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void testMergeWithEmptyListB() {
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SinglyLinkedList listA = new SinglyLinkedList();
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SinglyLinkedList listB = new SinglyLinkedList(); // Empty listB
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listA.insert(2);
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listA.insert(4);
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listA.insert(6);
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SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
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assertEquals("2->4->6", mergedList.toString(), "Merged list should match listA when listB is empty.");
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}
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@Test
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void testMergeWithBothEmptyLists() {
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SinglyLinkedList listA = new SinglyLinkedList(); // Empty listA
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SinglyLinkedList listB = new SinglyLinkedList(); // Empty listB
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SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
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assertEquals("", mergedList.toString(), "Merged list should be empty when both input lists are empty.");
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}
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@Test
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void testMergeWithDuplicateValues() {
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SinglyLinkedList listA = new SinglyLinkedList();
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SinglyLinkedList listB = new SinglyLinkedList();
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listA.insert(1);
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listA.insert(3);
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listA.insert(5);
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listB.insert(1);
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listB.insert(4);
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listB.insert(5);
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SinglyLinkedList mergedList = MergeSortedSinglyLinkedList.merge(listA, listB);
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assertEquals("1->1->3->4->5->5", mergedList.toString(), "Merged list should include duplicate values in sorted order.");
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}
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@Test
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void testMergeThrowsExceptionOnNullInput() {
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SinglyLinkedList listA = null;
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SinglyLinkedList listB = new SinglyLinkedList();
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listB.insert(1);
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listB.insert(2);
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SinglyLinkedList finalListA = listA;
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SinglyLinkedList finalListB = listB;
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assertThrows(NullPointerException.class, () -> MergeSortedSinglyLinkedList.merge(finalListA, finalListB), "Should throw NullPointerException if listA is null.");
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listA = new SinglyLinkedList();
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listB = null;
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SinglyLinkedList finalListA1 = listA;
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SinglyLinkedList finalListB1 = listB;
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assertThrows(NullPointerException.class, () -> MergeSortedSinglyLinkedList.merge(finalListA1, finalListB1), "Should throw NullPointerException if listB is null.");
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}
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}

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