Cần giúp đỡ về đề bài liên quan đến cấu trúc dữ liệu stack

Nguyen_Cong_Cuong · September 2, 2020, 11:17am

Đây là một bài cơ bản về stack (lv1) trên uva, nhưng mình đọc qua 4 5 lần rồi cũng ko hiểu tại sao kết quả lại ra yes hay no. Nên mình lên đây mong mọi người có thể giải thích giúp mình.

Đề bài gốc:
https://onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=24&page=show_problem&problem=455

There is a famous railway station in PopPush City. Country there is incredibly hilly. The station
was built in last century. Unfortunately, funds were extremely limited that time. It was possible to
establish only a surface track. Moreover, it turned out that the station could be only a dead-end one
(see picture) and due to lack of available space it could have only one track.

The local tradition is that every train arriving from the direction A continues in the direction
B with coaches reorganized in some way. Assume that the train arriving from the direction A has
N ≤ 1000 coaches numbered in increasing order 1, 2, . . . , N. The chief for train reorganizations must
know whether it is possible to marshal coaches continuing in the direction B so that their order will
be a1.a2, . . . , aN . Help him and write a program that decides whether it is possible to get the required
order of coaches. You can assume that single coaches can be disconnected from the train before they
enter the station and that they can move themselves until they are on the track in the direction B. You
can also suppose that at any time there can be located as many coaches as necessary in the station.
But once a coach has entered the station it cannot return to the track in the direction A and also once
it has left the station in the direction B it cannot return back to the station.

Input
The input file consists of blocks of lines. Each block except the last describes one train and possibly
more requirements for its reorganization. In the first line of the block there is the integer N described
above. In each of the next lines of the block there is a permutation of 1, 2, . . . , N. The last line of the
block contains just ‘0’.
The last block consists of just one line containing ‘0’.

Output
The output file contains the lines corresponding to the lines with permutations in the input file. A line
of the output file contains ‘Yes’ if it is possible to marshal the coaches in the order required on the
corresponding line of the input file. Otherwise it contains ‘No’. In addition, there is one empty line after
the lines corresponding to one block of the input file. There is no line in the output file corresponding
to the last “null” block of the input file.

Sample Input
5
1 2 3 4 5
5 4 1 2 3
0
6
6 5 4 3 2 1
0
0

Sample Output
Yes
No
Yes

nitro2 · September 2, 2020, 10:30pm

Mới đầu đọc đề không có tấm hình không hiểu gì hết :3
Mod nào vào Upload hình giúp bạn đi

Tóm tắt: Station ga tàu chỉ có 1 đường ray, đoàn tàu từ A trước khi vào ga có thể tách ra từng toa hoặc đi chung, rồi chạy ngược lại ra B. Hỏi có cách sắp xếp nào không thể thực hiện được không. Mặc định thứ tự toa lúc ở A là 1 2 3 … N
Ở đây có 3 test cases, format test case kiểu này hơi biến thái xíu nên nhìn hoài mới ra test case.
Cụ thể:

case 1: N=5, expected output= [1 2 3 4 5]
Từng toa từ 1 tới 5 lần lượt tách ra khỏi đoàn tàu rồi vào Station, rồi chạy qua B để ghép lại.
case 2: N=5, expected output [ 5 4 1 2 3]
Để ý thấy 5 ở đầu, do đó 5 phải là thằng cuối cùng từ A vào Station và là thằng đầu tiên rời Station đến B. Tương tự với 4. Tuy nhiên để 1 xếp trước 2 và 3 thì 1 phải tách toa ở A và chạy qua B trước. Trường hợp này không khả thi vì mâu thuẫn với điều kiện toa 5.
case 3: N=6, expected output [ 6 5 4 3 2 1]
Cái này thuần túy là đảo toa thôi, 1 vào trước ra sau.

Tóm lại, cái Station 1 đường ray này thực chất là 1 dạng thực tế của stack để thực hiện First in last out (FILO).