/// <summary>
/// Checks to see if there are circular references in a linked list, starting with the provided linked list node.
/// </summary>
public static bool HasCycles <T>(QuickLinkedListNode <T> node)
{
if (!CanContinue(node, node))
{
return(false); // Node is null and so there are no circular references.
}
return(FloydsCycleAlgoritym(node, node.Next.Next));
}
/// <summary>
/// Use the same node as tortoise and hair to start. Uses Floyds Cycle Algorithm explained in
/// https://en.wikipedia.org/wiki/Cycle_detection#Tortoise_and_hare
/// </summary>
/// <returns>True if cycle exists</returns>
private static bool FloydsCycleAlgoritym <T>(QuickLinkedListNode <T> tortoise, QuickLinkedListNode <T> hair)
{
if (!CanContinue(tortoise, hair)) // One or Both Racers found the end of the list
{
return(false);
}
if (tortoise == hair) // The two racers caught up to each other, indicating a cyclical loop in the linked list
{
return(true);
}
return(FloydsCycleAlgoritym(tortoise.Next, hair.Next.Next)); // Continue the race with the hair going twice as fast as the tortoise
}
/// <summary>
/// Creates a three linked list with the last linked to the first
/// </summary>
public static QuickLinkedListNode <int> GetLinkedListWithCyclicalReferences()
{
QuickLinkedListNode <int> firstNode = GetLinkedList();
QuickLinkedListNode <int> node = firstNode;
while (node.Next != null) // Retrieve last node in the list
{
node = node.Next;
}
node.Next = firstNode; // Create cyclical loop
return(firstNode);
}
/// <summary>
/// Checks the links of the nodes to verify that Floyds Cycle Algorithm can continue
/// </summary>
private static bool CanContinue <T>(QuickLinkedListNode <T> tortoise, QuickLinkedListNode <T> hair)
{
if (tortoise == null || hair == null) // One of the two racers have found the end of the list
{
return(false);
}
if (hair.Next == null) // Hair is twice as fast. Therefor, we need to check the node that he skips to see if it is the end of the list
{
return(false);
}
return(true); // Both tortoise and hair can safely continue forward
}
/// <summary>
/// Returns three linked list with no cyclical references
/// </summary>
public static QuickLinkedListNode <int> GetLinkedList()
{
QuickLinkedListNode <int> firstNode = new QuickLinkedListNode <int>();
firstNode = new QuickLinkedListNode <int>()
{
Value = 3,
Next = new QuickLinkedListNode <int>()
{
Value = 2,
Next = new QuickLinkedListNode <int>()
{
Value = 1,
}
}
};
return(firstNode);
}
