A.S->top!=0 B.S->top=0 C.S->top!=n D.S->top=n
A.k B.n-k-1 C.n-k+1 D.不確定
A.a,b,c,d,e B.d,e,c,b,a C.d,c,e,a,b D.e,d,c,b,a