Identify the top
Updated
The top is the only exposed value. In [4, 1, 9] the top is 9, not 4. Bottom is index 0 in this writing; top is the last index.
What to focus on
You name the top in O(1). You do not scan. After a push the new value is the top. After a pop the previous second-from-top becomes the top.
Worked examples
three items
a = [4, 1, 9]- last slot is 9
Result: top is 9
one item
a = [7]- the only item is the top
Result: top is 7
after push
a = [4, 1], push 9- 9 is the new last slot
Result: top is 9
Try it
Write top(a). a is the stack, written [bottom, ..., top]. Return the top value. Do not pop. On [4, 1, 9] return 9. On [7] return 7. After push 9 onto [4, 1], top is 9.
The stack is non-empty. Values are integers.