AddParticipants works perfectly, which leads to the Remove function to work perfectly as well

Marathons/Marathon.cs

@@ -30,12 +30,29 @@ public sealed class Marathon
         Node middleNode = new Node(participant);
         Node? leftNode = GetNodeByIndex(insertIndex - 1);
         Node? rightNode = GetNodeByIndex(insertIndex);
-        if (leftNode == null || rightNode == null)
+        if (leftNode == null && rightNode == null)
         {
             _head = middleNode;
             _tail = middleNode;
             return;
         }
+        if(leftNode == null && rightNode != null)
+        {
+            _head = middleNode;
+            MoveNode(middleNode, rightNode);
+            return;
+        }
+        if(rightNode == null && leftNode != null)
+        {
+            _tail = middleNode;
+            MoveNode(leftNode, middleNode);
+            return;
+        }
+        
+        if(rightNode == null || leftNode == null)
+        {
+            return;
+        }
         MoveNode(leftNode, rightNode, middleNode);
     }
 
@@ -100,14 +117,14 @@ public sealed class Marathon
         //check if the participant is the first or last
         int result = _head.Data?.CompareTo(participant) ?? 0;
         // If the participant is the first or can be placed before the first
-        if (result <= 0)
+        if (result >= 0)
         {
             exactPosFound = result == 0;
             return 0;
         }
         // If the participant is the last or can be placed after the last
         result = _tail.Data?.CompareTo(participant) ?? 0;
-        if (result >= 0)
+        if (result <= 0)
         {
             exactPosFound = result == 0;
             return ParticipantCount;

Marathons/Participant.cs

@@ -24,7 +24,7 @@ public sealed class Participant
         // Check if the completion time is the same
         if(CompletionTime == participant.CompletionTime)
         {
-            return StartNo < participant.StartNo ? -1 : 1;
+            return StartNo > participant.StartNo ? -1 : 1;
         }
         
         return CompletionTime < participant.CompletionTime ? -1 : 1;