大家好,我正在学习决赛,我不能解决这个问题：
Suppose that a client performs an intermixed sequence of stack push and pop operations. The push operations push the integers 0 through 9 in order on to the stack; the pop operations print out the return value. Which of the following sequences could not occur?
(a) 4 3 2 1 0 9 8 7 6 5
(b) 2 1 4 3 6 5 8 7 9 0
(c) 0 4 6 5 3 8 1 7 2 9
(d) 4 6 8 7 5 3 2 9 1 0
(e) All of these sequences are possible
答案是C,但我不确定如何得出这个结论
解决方法
好的,所以程序总是以这个顺序按0-9,所以看看每一行,我们找出推动和弹出的顺序
**The first line.** - Stack is Pushes 0,1,2,3,4 - [0,4] Pops 4,0 - [] Pushes 5,6,7,8,9 - [5,9] Pops 9,5 - [] **Second line** - Stack is Pushes 0,2 - [0,2] Pops 2,1 - [0] Pushes 3,4 - [0,4] Pops 4,3 - [0] Pushes 5,6 - [0,5,6] Pops 6,5 - [0] Pushes 7,8 - [0,8] Pops 8,7 - [0] Pushes 9 - [0,9] Pops 9,0 - [] **Third line** - Stack is Pushes 0 - [0] Pops 0 - [] Pushes 1,4 - [1,- [1,3] Pushes 5,6 - [1,3 - [1,2] Pushes 7,8 - [1,8] Pops 8 - [1,7] Pops ?
下一个pop必须是7,因为它在8之前被推,它不能是1.
