Add DeleteFirst and DeleteLast to SinglyLinkedList (#450)

Author: Ozzy-ZY

DataStructures.Tests/LinkedList/LinkedListTests.cs

@@ -106,4 +106,71 @@ public static void RemoveItemsFromList()
         Assert.That(l3RemoveSucess, Is.False);
         Assert.That(nonExistantRemoveSucess, Is.False);
     }
+
+    [Test]
+    public static void DeleteFirstFromList()
+    {
+        // Arrange
+        var testObj = new SinglyLinkedList<string>();
+        _ = testObj.AddLast("H");
+        _ = testObj.AddLast("E");
+        _ = testObj.AddLast("L");
+        _ = testObj.AddLast("L");
+        _ = testObj.AddLast("O");
+
+        // Act
+        var deleteSuccess = testObj.DeleteFirst();
+
+        // Assert
+        Assert.That(deleteSuccess, Is.True);
+        Assert.That(4, Is.EqualTo(testObj.Length()));
+        Assert.That("E", Is.EqualTo(testObj.GetElementByIndex(0)));
+    }
+
+    [Test]
+    public static void DeleteFirstFromEmptyList()
+    {
+        // Arrange
+        var testObj = new SinglyLinkedList<string>();
+
+        // Act
+        var deleteSuccess = testObj.DeleteFirst();
+
+        // Assert
+        Assert.That(deleteSuccess, Is.False);
+    }
+
+    [Test]
+    public static void DeleteLastFromList()
+    {
+        // Arrange
+        var testObj = new SinglyLinkedList<string>();
+        _ = testObj.AddLast("H");
+        _ = testObj.AddLast("E");
+        _ = testObj.AddLast("L");
+        _ = testObj.AddLast("L");
+        _ = testObj.AddLast("O");
+
+        // Act
+        var deleteSuccess = testObj.DeleteLast();
+
+        // Assert
+        Assert.That(deleteSuccess, Is.True);
+        Assert.That(4, Is.EqualTo(testObj.Length()));
+        Assert.That("L", Is.EqualTo(testObj.GetElementByIndex(testObj.Length() - 1)));
+    }
+
+    [Test]
+    public static void DeleteLastFromEmptyList()
+    {
+        // Arrange
+        var testObj = new SinglyLinkedList<string>();
+
+        // Act
+        var deleteSuccess = testObj.DeleteLast();
+
+        // Assert
+        Assert.That(deleteSuccess, Is.False);
+    }
+
 }

DataStructures/LinkedList/SinglyLinkedList/SinglyLinkedList.cs

@@ -153,4 +153,51 @@ public bool DeleteElement(T element)
 
         return false;
     }
+
+    /// <summary>
+    /// Deletes the first element of the list.
+    /// </summary>
+    /// <returns> true if the operation is successul.</returns>
+    public bool DeleteFirst()
+    {
+        // checks if the List is empty
+        if(Head is null)
+        {
+            return false;
+        }
+
+        // if not, the head is overwritten with the next element and the old head is deleted
+        Head = Head.Next;
+        return true;
+    }
+
+    /// <summary>
+    /// Deletes the last element of the list.
+    /// </summary>
+    /// <returns> returns true if the operation is successful. </returns>
+    public bool DeleteLast()
+    {
+        // checks if the List is empty
+        if(Head is null)
+        {
+            return false;
+        }
+
+        // checks if the List has only one element
+        if(Head.Next is null)
+        {
+            Head = null;
+            return true;
+        }
+
+        // if not, iterates through the list to the second last element and deletes the last one
+        SinglyLinkedListNode<T>? secondlast = Head;
+        while(secondlast.Next?.Next is not null)
+        {
+            secondlast = secondlast.Next;
+        }
+
+        secondlast.Next = null;
+        return true;
+    }
 }