// Merge two subtrees together, returning the one that came out on top.
// For a pairing heap, merging consists of adding the larger node
// to the sub list of the smaller.
private HeapNode <T> merge(HeapNode <T> a, HeapNode <T> b)
{
if (compare(a.getNode(), b.getNode()) <= 0) // a <= b (swap a and b)
{
HeapNode <T> tmp = a;
a = b; b = tmp;
}
// Here we assume that a > b, therefore a is added to b's children
// Remove a from any child list it might be in.
a.extract();
// Link a into b's child list
b.addChild(a);
return(b);
}