資料結構與演算法(A)

(5 points) There is a large enough array $A$ with indices $p$ and $q$ which are initialized by 0, and functions $f(x)$ and $g()$ are defined as follows. Array $A$ and indices $p$ and $q$ can only be accessed in the functions $f(x)$ and $g()$.

f(x):
    p ← p + 1
    A[p] ← x

g():
    x ← A[p]
    p ← p - 1
    return x

Which data structure is operated by those functions?