本题考察知识点是:栈的性质是后进先出 所以:分析如下 栈里的元素最多为2+1=3 进栈元素                         栈里元素(栈底->栈顶) 1:a进栈                            a 2:b进栈 出栈                    a 3:c进栈                            a c 4:d进栈 出战                    a c 5:c出栈                            a 6:e进栈                            a e 7:f进栈 出栈                     a e 8:e出栈                            a 9:a出栈