// Return true if the list contains a loop.
public bool ContainsLoop()
{
// Lists with 0 or 1 items don't contain loops.
if ((this.Next == null) ||
(this.Next.Next == null))
{
return(false);
}
ValueCell slowCell = this.Next;
ValueCell fastCell = slowCell.Next;
// Start the cells running.
for (; ;)
{
if (slowCell == fastCell)
{
return(true);
}
slowCell = slowCell.Next;
fastCell = fastCell.Next;
if (fastCell.Next == null)
{
return(false);
}
fastCell = fastCell.Next;
if (fastCell.Next == null)
{
return(false);
}
}
}
// Make a list that contains a loop.
private void BuildList()
{
// Make some cells.
ValueCell[] cells = new ValueCell[NumCells];
for (int i = 0; i < cells.Length; i++)
{
cells[i] = new ValueCell();
cells[i].Value = i + 1;
}
// Link the cells.
for (int i = 0; i < cells.Length - 1; i++)
{
cells[i].Next = cells[i + 1];
}
// Make the loop.
cells[cells.Length - 1].Next = cells[LoopCell];
// Prepare the sentinel.
TopCell = new ValueCell()
{
Next = cells[0]
};
}
// Display the list in the indicated ListBox.
private void DisplayList(ListBox lst)
{
// The maximum number of cells we will list.
const int maxCells = 14;
// Display list.
lst.Items.Clear();
int cellNum = 0;
for (ValueCell cell = TopCell.Next; cell != null; cell = cell.Next)
{
lst.Items.Add(cell.Value);
// Stop after a while.
if (++cellNum > maxCells)
{
lst.Items.Add("...");
break;
}
}
}
// If this list after this cell contains a loop,
// break it so we have a normal list.
public void BreakLoop()
{
if (!ContainsLoop())
{
return;
}
ValueCell slowCell = this.Next;
ValueCell fastCell = slowCell.Next;
// Start the cells running and see where they meet.
for (; ;)
{
if (slowCell == fastCell)
{
break;
}
slowCell = slowCell.Next;
fastCell = fastCell.Next;
fastCell = fastCell.Next;
}
// Start slowCell again at the beginning and
// run the two at the same speed until their Next
// cells are the same.
slowCell = this;
while (slowCell.Next != fastCell.Next)
{
slowCell = slowCell.Next;
fastCell = fastCell.Next;
}
// At this point, slowCell.Next is the first cell in the
// loop and lastCell.Next is the last cell in the loop.
fastCell.Next = null;
}