Add tests for SinglyLinkedList (#3913)

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

@@ -54,6 +54,24 @@ public class SinglyLinkedList extends Node {
         return false;
     }
 
+    /**
+     * Return the node in the middle of the list
+     * If the length of the list is even then return item number length/2
+     * @return middle node of the list
+     */
+    public Node middle() {
+        if (head == null) {
+            return null;
+        }
+        Node firstCounter = head;
+        Node secondCounter = firstCounter.next;
+        while (secondCounter != null && secondCounter.next != null) {
+            firstCounter = firstCounter.next;
+            secondCounter = secondCounter.next.next;
+        }
+        return firstCounter;
+    }
+
     /**
      * Swaps nodes of two given values a and b.
      *

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

@@ -0,0 +1,102 @@
+package com.thealgorithms.datastructures.lists;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class SinglyLinkedListTest {
+
+    /**
+     * Initialize a list with natural order values with pre-defined length
+     * @param length
+     * @return linked list with pre-defined number of nodes
+     */
+    private SinglyLinkedList createSampleList(int length) {
+        List<Node> nodeList = new ArrayList<>();
+        for (int i = 1; i <= length; i++) {
+            Node node = new Node(i);
+            nodeList.add(node);
+        }
+
+        for (int i = 0; i < length - 1; i++) {
+            nodeList.get(i).next = nodeList.get(i+1);
+        }
+
+        return new SinglyLinkedList(nodeList.get(0), length);
+    }
+
+    @Test
+    void detectLoop() {
+        //List has cycle
+        Node firstNode = new Node(1);
+        Node secondNode = new Node(2);
+        Node thirdNode = new Node(3);
+        Node fourthNode = new Node(4);
+
+        firstNode.next = secondNode;
+        secondNode.next = thirdNode;
+        thirdNode.next = fourthNode;
+        fourthNode.next = firstNode;
+
+        SinglyLinkedList listHasLoop = new SinglyLinkedList(firstNode, 4);
+        assertTrue(listHasLoop.detectLoop());
+
+        SinglyLinkedList listHasNoLoop = createSampleList(5);
+        assertFalse(listHasNoLoop.detectLoop());
+    }
+
+    @Test
+    void middle() {
+        int oddNumberOfNode = 7;
+        SinglyLinkedList list = createSampleList(oddNumberOfNode);
+        assertEquals(oddNumberOfNode/2 + 1, list.middle().value);
+        int evenNumberOfNode = 8;
+        list = createSampleList(evenNumberOfNode);
+        assertEquals(evenNumberOfNode/2, list.middle().value);
+
+        //return null if empty
+        list = new SinglyLinkedList();
+        assertNull(list.middle());
+
+        //return head if there is only one node
+        list = createSampleList(1);
+        assertEquals(list.getHead(), list.middle());
+    }
+
+    @Test
+    void swap() {
+        SinglyLinkedList list = createSampleList(5);
+        assertEquals(1, list.getHead().value);
+        assertEquals(5, list.getNth(4));
+        list.swapNodes(1,5);
+        assertEquals(5, list.getHead().value);
+        assertEquals(1, list.getNth(4));
+    }
+
+    @Test
+    void clear() {
+        SinglyLinkedList list = createSampleList(5);
+        assertEquals(5, list.size());
+        list.clear();
+        assertEquals(0, list.size());
+        assertTrue(list.isEmpty());
+    }
+
+    @Test
+    void search() {
+        SinglyLinkedList list = createSampleList(10);
+        assertTrue(list.search(5));
+        assertFalse(list.search(20));
+    }
+
+    @Test
+    void deleteNth() {
+        SinglyLinkedList list = createSampleList(10);
+        assertTrue(list.search(7));
+        list.deleteNth(6); //Index 6 has value 7
+        assertFalse(list.search(7));
+    }
+}