// O(|head1|+|head2|) runtime, O(1) space
public static LinkedList.Node FindIntersection(LinkedList.Node head1, LinkedList.Node head2)
{
// If either head node is null, throw exception.
if (head1 == null)
{
throw new System.ArgumentNullException("head1");
}
if (head2 == null)
{
throw new System.ArgumentNullException("head2");
}
// Create array of NodeCounts to organize data.
var ncArray = new NodeCount[2]
{
new NodeCount()
{
Node = head1
},
new NodeCount()
{
Node = head2
}
};
// Calculate the length and terminating node of each list.
for (int i = 0; i < ncArray.Length; i++)
{
ncArray[i].TerminalNode = ncArray[i].Node;
while (ncArray[i].TerminalNode.Next != null)
{
ncArray[i].Count++;
ncArray[i].TerminalNode = ncArray[i].TerminalNode.Next;
}
}
// If the terminating nodes of the lists are not the same node, they do not intersect.
if (!ncArray[0].TerminalNode.Equals(ncArray[0].TerminalNode))
{
return(null);
}
// Sort ncArray by length of each list.
ncArray = ncArray.OrderBy(x => x.Count).ToArray <NodeCount>();
// Trim the start of the longer list until they are the same length.
while (ncArray[1].Count > ncArray[0].Count)
{
ncArray[1].Node = ncArray[1].Node.Next;
ncArray[1].Count--;
}
// Check node-by-node until the intersecting node is found.
while (ncArray[0].Node.Equals(ncArray[1].Node))
{
ncArray[0].Node = ncArray[0].Node.Next;
ncArray[1].Node = ncArray[1].Node.Next;
}
return(ncArray[0].Node);
}
