&扇 A stack,concrete object 102 capacity (array_up) count “Push”operation: count:count 1 representation [count]:=x representation “Push”operation: representation [free]:=x (array_down) free:=free-1 free 1 representation “Push"operation: new new (n) n item previous n.item:=x n.previous:=last item previous (linked) previous head:=n itemA stack, concrete object 20 20 count representation (array_up) capacity representation [count] := x free (array_down) 1 representation n new (linked) item item previous item previous previous “Push” operation: count := count + 1 representation [free] := x “Push” operation: free := free - 1 new (n) n.item := x “Push” operation: n.previous := last head := n