Add function doesnt work as supposed at 3+ participants

Marathons/Marathon.cs

@@ -2,7 +2,8 @@
 
 public sealed class Marathon
 {
-    //private Node? _head;
+    private Node? _head;
+    private Node? _tail;
     public readonly string City;
     public readonly DateOnly Date;
     public int ParticipantCount { get; private set; }
@@ -13,16 +14,61 @@ public sealed class Marathon
         Date = date;
     }
 
+    /// <summary>
+    /// Adds a participant to the list
+    /// </summary>
+    /// <param name="participant">The participant</param>
     public void AddParticipant(Participant participant)
     {
+        int insertIndex = GetIndex(participant, out bool exactPosFound);
+        if (exactPosFound)
+        {
+            return;
+        }
         
+        ParticipantCount++;
+        Node middleNode = new Node(participant);
+        Node? leftNode = GetNodeByIndex(insertIndex - 1);
+        Node? rightNode = GetNodeByIndex(insertIndex);
+        if (leftNode == null || rightNode == null)
+        {
+            _head = middleNode;
+            _tail = middleNode;
+            return;
+        }
+        MoveNode(leftNode, rightNode, middleNode);
     }
 
-    public bool RemoveParticipant(int index)
+    public bool RemoveParticipant(int startNo)
     {
-        return false;
-    }
+        Node? nodeToRemove = GetNodeByStartNo(startNo);
+        if (nodeToRemove == null)
+        {
+            return false;
+        }
 
+        ParticipantCount--;
+        Node? leftNode = GetNodeByIndex(GetIndex(nodeToRemove.Data!, out _) - 1);
+        // some explanation here |↑ node of → |↑ index of → |↑ Participant
+        Node? rightNode = nodeToRemove.Next;
+        
+        // If nodeToRemove is the first node
+        if (leftNode == null)
+        {
+            _head = rightNode;
+            return true;
+        }
+        // If nodeToRemove is the last node
+        if (rightNode == null)
+        {
+            _tail = leftNode;
+            return true;
+        }
+        
+        MoveNode(leftNode, rightNode);
+        return true;
+    }
+    
     public string[] GetResultList()
     {
         return Array.Empty<string>();
@@ -32,4 +78,138 @@ public sealed class Marathon
     {
         return $"{City} marathon on {Date.ToString(Const.Culture)}";
     }
+    
+    /// <summary>
+    /// Returns the index of the participant in the list
+    /// It's a merged method that can be used to determine the index of a participant,
+    /// or the index where the participant can be placed
+    /// Should be efficient for VERY large lists
+    /// </summary>
+    /// <param name="participant">the participant you want to check</param>
+    /// <param name="exactPosFound">true: found exact location; false: no exact location/doesn't exist</param>
+    /// <returns>The index where the participant is or might be</returns>
+    private int GetIndex(Participant participant, out bool exactPosFound)
+    {
+        //check if the list is empty
+        if (_head is null || _tail is null)
+        {
+            exactPosFound = false;
+            return 0;
+        }
+        
+        //check if the participant is the first or last
+        int result = _head.Data?.CompareTo(participant) ?? 0; // why ?? are you confused? you sure its not null??
+                                                              // haha, you sure??
+                                                              // I'm sure, I'm sure
+                                                              // haha, okay, I trust you
+                                                              // ok, i should stop talking to co-pilot now
+        // If the participant is the first or can be placed before the first
+        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)
+        {
+            exactPosFound = result == 0;
+            return ParticipantCount;
+        }
+        
+        // Half the list and check both sides
+        int left = 0;
+        int right = ParticipantCount;
+        while (left < right)
+        {
+            int middle = (left + right) / 2;
+            Node? current = _head;
+            for (int i = 0; i < middle; i++)
+            {
+                current = current?.Next;
+            }
+            result = current?.Data?.CompareTo(participant) ?? 0;
+            if (result == 0)
+            {
+                exactPosFound = true;
+                return middle;
+            }
+            if (result < 0)
+            {
+                left = middle + 1;
+            }
+            else
+            {
+                right = middle;
+            }
+        }
+        
+        // If not in the list, return the index where it should be
+        exactPosFound = false;
+        return left;
+    }
+
+    /// <summary>
+    /// Makes the connection to the specified node
+    /// A -> B -> C
+    /// If no middle node specified, then it will be A -> C
+    /// </summary>
+    /// <param name="startingNode">The starting Node, or A node</param>
+    /// <param name="targetNode">The targetNode, or the B node if no middle node specified, else C node</param>
+    /// <param name="middleNode">The middle node or the B node</param>
+    private void MoveNode(Node startingNode, Node targetNode, Node? middleNode = null)
+    {
+        if(middleNode is null)
+        {
+            startingNode.Next = targetNode;
+            return;
+        }
+
+        if(startingNode == targetNode)
+        {
+            _tail = middleNode;
+        }
+        else middleNode.Next = targetNode;
+        startingNode.Next = middleNode;
+    }
+    
+    Node? GetNodeByIndex(int index)
+    {
+        if(index <= 0)
+        {
+            return _head;
+        }
+        if(index >= ParticipantCount)
+        {
+            return _tail;
+        }
+        
+        Node? current = _head;
+        for (int i = 0; i < index; i++)
+        {
+            current = current?.Next;
+        }
+
+        return current;
+    }
+    
+    Node? GetNodeByStartNo(int startNo)
+    {
+        Node? current = _head;
+        for (int i = 0; i < ParticipantCount; i++)
+        {
+            if (current == null)
+            {
+                return null;
+            }
+            
+            if (current.Data?.StartNo == startNo)
+            {
+                return current;
+            }
+            current = current.Next;
+        }
+        
+        return null;
+    }
 }

Marathons/Node.cs

@@ -1,7 +1,8 @@
 namespace Marathons;
 
-public sealed class Node(double? value)
+public sealed class Node(Participant? value)
 {
-    public double? Data { get; } = value;
+    //accept anything
+    public Participant? Data { get; } = value;
     public Node? Next { get; set; }
 }