Concatenar dos pilas (una pila sobre otra)

I understand how to do this with a loop S1 and S2 (in pseudocode)

Loop (NOT emptyStack(S2))
     pop(S2, dataptr)
     push (temp, dataptr)
end loop 
Loop (NOT emptyStack(temp))
     pop (temp dataptr)
     push(S1, dataptr)
end loop 

How would you do this without a loop (big O constant)? With queues it is easy as all you have to do is move the rear pointer of q1 to the front of q2 and link them.

C like pseudocode would be great! Thank you very much.