cs314cl p. 36

Suppose we perform the following operations on a Stack: (push 1) (push 2) (push 3) (pop) (pop) (push 4) (pop) (pop)
What is the order of the items that come out?

A: 1, 2, 3, 4

B: 4, 3, 2, 1

C: 2, 3, 1, 4

D: 3, 2, 4, 1

E: 2, 3, 4, 1

Answer: D

push and pop both operate at the top of the stack only. If we use a list as the stack, with the top of stack at the left, the successive operations are:

initial: stack = ()
(push 1) stack = (1)
(push 2) stack = (2 1)
(push 3) stack = (3 2 1)
(pop) stack = (2 1), 3 is produced
(pop) stack = (1), 2 is produced
(push 4) stack = (4 1)
(pop) stack = (1), 4 is produced
(pop) stack = (), 1 is produced

Contents Page-10 Prev Next Page+10 Index