// The user must call this function to signal that a key has been decresed.
// The function will then remove the node and subnodes from the heap, then
// merge them back in. The user gets the heap node reference from insert.
public void decreaseKey(HeapNode <T> node)
{
if (node == top || node == null || top == null)
{
return;
}
node.extract();
if (merge(top, node) == node)// Node has become new top
{
top = node;
}
}