Push one value
Updated
Push adds a value as the new top. Everything already on the stack stays, now one slot deeper. Size increases by 1.
What to focus on
After push(x), peek is x. The old top is still there, buried. Empty push makes a one-item stack. You do not shift buried values by hand. The structure does that.
Worked examples
onto three
a = [4, 1, 9], push 8- 8 becomes top
- 9 is now under 8
Result: [4, 1, 9, 8] · top 8
onto one
a = [7], push 3- 3 on top of 7
Result: [7, 3] · top 3
onto empty
a = [], push 4- 4 is top and bottom
Result: [4]
Try it
Write push(a, x). a is the stack, written [bottom, ..., top]. x is the integer to add as the new top. Add x on top. Size increases by 1. Peek must be x. Push 8 onto [4, 1, 9] → [4, 1, 9, 8]. Push 3 onto [7] → [7, 3]. Push 4 onto [] → [4].
Values are integers. You may push onto an empty stack.