A stack is an abstract data type — a mathematical or logical model specifying only the available operations (push, pop, top, isEmpty) and their constant-time requirement, not their realization — defined as a list constrained so that insertion and deletion occur one element at a time and only at a single end called the top. Adding that single constraint to any list realization yields a stack, and the array realization does so by pairing a fixed array with an integer index marking the top, using -1 as the sentinel for an empty stack so push increments before writing and pop merely decrements, both in O(1); vacated cells need no clearing since they are unreachable through the interface and will be overwritten. The realization's characteristic limitation is overflow when the array is exhausted, addressable either by rejecting the push or by the dynamic-array strategy of allocating a doubled array and copying, which costs O(n) in the worst case but amortizes to O(1) per push — placing this squarely in the data structures discipline's concern with separating an ADT's contract from the cost profile of its concrete implementations.
A stack is an abstract data type — a mathematical or logical model specifying only the available operations (push, pop, top, isEmpty) and their constant-time requirement, not their realization — defi…