13 對兩個空堆疊(empty stacks)S1 及 S2 分別執行下列步驟後,何者的最後一個 S2.pop( )之回傳值為 B?
(註:push 為加入一元素到 stack 之動作,pop 為由 stack 取出一元素之動作。)(A) S1.push
, S1.push
, S2.push
, S1.push(S2.pop( )), S2.push(S1.pop( )), S2.pop( )
(B) S1.push
, S1.push
, S2.push(S1.pop( )), S2.push
, S1.push(S2.pop( )), S2.pop( )
(C) S1.push
, S2.push(S1.pop( )), S1.push
, S2.push
, S1.push(S2.pop( )), S2.pop( )
(D) S2.push
, S1.push(S2.pop( )), S1.push
, S2.push(S1.pop( )), S1.push
, S2.pop( )