Fix red black tree for remove one and only element from tree.

Author: PatrykOlejniczak

DataStructures/Trees/RedBlackTree.cs

@@ -465,54 +465,61 @@ protected bool _remove(RedBlackTreeNode<TKey> nodeToDelete)
                 return false;
             }
 
-            // X it's node that will become move to original nodeToDelete position in the tree.
-            RedBlackTreeNode<TKey> x;
-
-            if (nodeToDelete.HasOnlyRightChild)
-            {
-                x = nodeToDelete.RightChild;
-                Transplant(nodeToDelete, nodeToDelete.RightChild);
-            }
-            else if (nodeToDelete.HasOnlyLeftChild)
+            if (!nodeToDelete.HasChildren)
             {
-                x = nodeToDelete.LeftChild;
-                Transplant(nodeToDelete, nodeToDelete.LeftChild);
+                Root = null;
             }
             else
             {
-                // Y it's node that will become move to original X position in the tree.
-                var y = (RedBlackTreeNode<TKey>)_findMinNode(nodeToDelete.RightChild);
-                x = y.RightChild;
+                // X it's node that will become move to original nodeToDelete position in the tree.
+                RedBlackTreeNode<TKey> x;
 
-                if (y.Parent == nodeToDelete)
+                if (nodeToDelete.HasOnlyRightChild)
                 {
-                    if (x != null)
-                    {
-                        x.Parent = y;
-                    }
+                    x = nodeToDelete.RightChild;
+                    Transplant(nodeToDelete, nodeToDelete.RightChild);
                 }
-                else
+                else if (nodeToDelete.HasOnlyLeftChild)
                 {
-                    Transplant(y, y.RightChild);
-                    y.RightChild = nodeToDelete.RightChild;
-                    y.RightChild.Parent = y;
+                    x = nodeToDelete.LeftChild;
+                    Transplant(nodeToDelete, nodeToDelete.LeftChild);
                 }
+                else
+                {
+                    // Y it's node that will become move to original X position in the tree.
+                    var y = (RedBlackTreeNode<TKey>) _findMinNode(nodeToDelete.RightChild);
+                    x = y.RightChild;
 
-                Transplant(nodeToDelete, y);
-                y.LeftChild = nodeToDelete.LeftChild;
-                y.LeftChild.Parent = y;
-                y.Color = nodeToDelete.Color;
+                    if (y.Parent == nodeToDelete)
+                    {
+                        if (x != null)
+                        {
+                            x.Parent = y;
+                        }
+                    }
+                    else
+                    {
+                        Transplant(y, y.RightChild);
+                        y.RightChild = nodeToDelete.RightChild;
+                        y.RightChild.Parent = y;
+                    }
 
-                if (Root == nodeToDelete)
-                {
-                    Root = y;
-                    Root.Parent = null;
+                    Transplant(nodeToDelete, y);
+                    y.LeftChild = nodeToDelete.LeftChild;
+                    y.LeftChild.Parent = y;
+                    y.Color = nodeToDelete.Color;
+
+                    if (Root == nodeToDelete)
+                    {
+                        Root = y;
+                        Root.Parent = null;
+                    }
                 }
-            }
 
-            if (nodeToDelete.Color == RedBlackTreeColors.Black)
-            {
-                _adjustTreeAfterRemoval(x);
+                if (nodeToDelete.Color == RedBlackTreeColors.Black)
+                {
+                    _adjustTreeAfterRemoval(x);
+                }
             }
 
             base._count--;

UnitTest/DataStructuresTests/RedBlackTreeTest.cs

@@ -374,6 +374,25 @@ public void Remove_ThrowExceptionWhenTryRemoveNonExistentNode()
             //TODO Create more specyfic exception type for this kind of errors, with inheritance from ArgumentException.
             Assert.Throws<Exception>(() =>redBlackTree.Remove(999));
         }
+
+        [Fact]
+        public void Remove_OneAndOnlyTreeNode()
+        {
+            var oneElementTree = new RedBlackTree<int>();
+            Assert.Equal(0, oneElementTree.Count);
+            Assert.Null(oneElementTree.Root);
+            Assert.True(oneElementTree.IsEmpty);
+
+            oneElementTree.Insert(1);
+            Assert.Equal(1, oneElementTree.Count);
+            Assert.NotNull(oneElementTree.Root);
+            Assert.False(oneElementTree.IsEmpty);
+
+            oneElementTree.Remove(1);
+            Assert.Equal(0, oneElementTree.Count);
+            Assert.Null(oneElementTree.Root);
+            Assert.True(oneElementTree.IsEmpty);
+        }
     }
 
     /// <summary>