12. Explain the following stack member functions used in C++.
(a) empty() (b) size() (c) pop()
(d) stack() (e) push() (f) top()
13. What is a queue? Explain how a queue is simulated in STL.
14. Discuss the following queue member functions used in C++.
(a) back() (b) push() (c) empty()
(d) queue() (e) front() (f) size()
(g) pop()
15. Explain how a priority_queue class is realised in STL.