資料結構與演算法(A)

(5 points) 承上題（陣列 $A$ 與索引 $p, q$ 初值皆 0，僅能由 $f, g$ 存取）。If functions $f(x)$ and $g()$ are changed as following:

f(x):
    A[q] ← x
    q ← q + 1

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

Which data structure is operated by those functions?