##### Canadian Computing Competition: 2018 Stage 1, Senior #3

A robot has stolen treasure from a factory and needs to escape without getting caught. The factory can be modelled by an by grid, where the robot can move up, down, left, or right.

Each cell of the grid is either empty, a wall, a camera, a conveyor, or the robot's initial position.
The robot can only walk on empty cells (denoted by `.`

) or conveyors. The first row, last row, first
column and last column of the grid consists of walls (denoted by `W`

), and there may be walls in
other cells.

Conveyors cause the robot to move in a specific direction, denoted by `L`

, `R`

, `U`

, `D`

for left, right, up,
down respectively. The robot is unable to move on its own while on a conveyor. It is possible that
the robot can become stuck forever on conveyors.

Cameras (denoted by `C`

) can see in all four directions up, down, left, and right, but cannot see
through walls. The robot will be caught if it is in the same cell as a camera or is seen by a camera
while not on a conveyor. Conveyors are slightly elevated, so the robot cannot be caught while on a
conveyor, but cameras can see empty cells on the other side of conveyors.

The robot is initially at the cell denoted by `S`

. The exit could be at any of the empty cells. For each
empty cell, determine the minimum number of steps needed for the robot to move there without
being caught, or determine that it is impossible to move there. A step consists of moving once up,
down, left or right. Being moved by a conveyor does not count as a step.

#### Input Specification

The first line of input contains two integers and . The next lines of
input will each contain characters, each of which is one of the eight characters `W`

, `.`

, `C`

, `S`

, `L`

, `R`

,
`U`

, or `D`

.

There will be exactly one `S`

character and at least one `.`

character. The first and last character of
every row and column will be `W`

.

For of the marks available, there are no cameras or conveyors.

For an additional of the marks available, there are no conveyors.

#### Output Specification

For each empty cell, print one line with one integer, the minimum number of steps for the robot to
move to this empty cell without being caught or `-1`

if it is impossible to move to this empty cell.

The output should be in row major order; the order of empty cells seen if the input is scanned line by line top-to-bottom and then left-to-right on each line. See the sample outputs for examples of row major order output.

#### Sample Input 1

```
4 5
WWWWW
W.W.W
WWS.W
WWWWW
```

#### Sample Output 1

```
-1
2
1
```

#### Explanation for Sample Output 1

The robot cannot move to the top left empty cell because it is blocked by walls.

The top right empty cell can be reached in steps and the bottom right empty cell can be reached in step.

#### Sample Input 2

```
5 7
WWWWWWW
WD.L.RW
W.WCU.W
WWW.S.W
WWWWWWW
```

#### Sample Output 2

```
2
1
3
-1
-1
1
```

#### Explanation for Sample Output 2

The empty cell to the immediate left of the robot is seen by the camera so the robot cannot move there.

The empty cell right below the `R`

conveyor is also seen by the camera as conveyors do not block
the sight of cameras.

Note that the robot can use the `U`

and `L`

conveyors to avoid getting caught by the camera.

If the robot moves to the `R`

conveyor, it will become stuck there forever.

## Comments

how do you do this problem?

this, and a lot of implementation details.

guess and check

Not sure anyone really needs this, but due to the amount of help I needed and to help with others who will come across this problem, I've compiled some ideas in case you need the help.

Issues I had when implementing:

Replacing camera squares: I initially thought that it was a great idea to replace empty spaces covered by cameras with other characters - however this proved to be troublesome. It is possible to make this method work, however personally, I think the more efficient method would be to store invalid squares separately. Here's the reason why - the rest is left as exercise to the reader: a camera covered square does not guarantee an open square

BFS implementation: If you are using BFS (which you probably should be) make sure that what you are checking is accurate. This includes checking both what square you are on and what square you are moving to. Hint (not given as full to stay an exercise to the reader): if you are using a visited array and cross an infinite loop of conveyors, how do you ensure you only go around a maximum of one time through the use of your visited?

Filling your array with a preliminary large int: This is only a note for C++ users and is in fact very minor. Using "memset" does not fill an array; instead use the "std::fill" function.

Two conveyor case: Once you have fixed up the conveyor loop, this likely isn't much of an issue but is still worthwhile to check: two consecutive conveyors is a bit of a corner case that may not work

Hopefully this leaves a lot of the problem as an exercise, but still helps in the case that you are severely stuck in finding corner cases - not all your issues might be considered here as I use C++, but maybe it can help out. I highly recommend also checking out other cases in the comments to help fix your work.

orz pavi

My solution is WAing on the 2nd test case of batch 5, but I'm not sure what's wrong. I have checked all related comments and am handling all edge cases I know of.

I've also tested my solution locally using the official test data and there was no difference between my output and the expected output (checked via

`diff`

), which leads me to suspect that my code is relying upon undefined behavior somewhere. Unfortunately, I'm not skilled enough to find it.If someone could help, that'd be great. Thanks!

The judge server compiles your program with -O2, which in your case seems to cause problems with g++; specifically on line 132 and 133 (storing your index in a variable rather than inline seems to fix it). Regardless though, submitting your original solution with clang++ seems to pass just fine.

Ugh. Thank you so much, really appreciate it. In hindsight, I'm actually somewhat surprised that it doesn't WA earlier / at all on Clang.

Anyone cannot pass batch 5 Considering this case

the answer should be

Also, guys, there is a very annoying test case where the conveyor forms a loop and the robot is stuck on conveyors forever. It is batch 3 test 2. I got maximum recursion depth exceeded.

How do I check the distances when using a BFS using a queue? I am currently storing the distances inside structs but I have to reset the whole matrix every time I BFS and I think it's causing TLE. Also, I don't know how to make it so that the robot prioritizes taking the path with a bunch of conveyors because my program treats conveyors as normal points but it doesn't add one to the distance so sometimes it thinks that the shortest path is the path with a bunch of dots instead of a path with a conveyor.

EDIT: I used while loops to fix the shortest path conveyor issue but I still don't know how to store distances the optimal way. Could someone guide me? Thanks.

For the distances, you can store a

`dist`

variable inside your struct, and every time you move to a cell, set that cell's distance to the current distance travelled.Also for conveyors, you can think of them as moving twice in the same step. And since this is a BFS, you are guaranteed to find the shortest path

Yea that's what I did, I thought it would TLE but I only have 1 TLE and that's on batch 1 so I think storing the dist inside the structs was actually fine. I'm still not sure what to do about that 1 TLE though. Thanks for the response.

Edit: Nevermind I finally managed to solve it by memoizing any dots passed by on any BFS so that if you BFS them again there is no need to call the function and instead you can just use an array lookup.

I'm getting WA on Batch #3 Case #2 and Batch #5 Case #2. I read all the comments and made sure my code works for all special cases like when you start next to a camera, or a never ending conveyor. Is there something else I'm missing?

Consider the following test case:

The output should be all -1s.

Oh I see what I did wrong! I iterated through each camera, and then directly turned the invalid positions into walls.

I should of stored the invalid positions in a separate array instead of modifying the grid directly.

Thanks!

For anyone stuck on Batch 5 Case 2, make sure you have some code dealing with if the robot finds himself stuck on two conveyors and cannot escape (WLRW). (Besides also having some code to check if you spawn next to a camera) That was the issue for me.

can someone assist, I am getting WA on case #2 of the last bacth

Did you check for the case where you spawn right beside the camera?

thanks for your help. that was the only case i was missing

Hello, I'm getting WA for Batch 5 Case 2, and I couldn't find any bugs with my program. Are there any special cases/pitfalls that I'm unaware of? I already included code to handle the situation where the robot spawns in the sight of a camera, and I couldn't think of any other corner cases.

Thanks for helping!

Consider The Case:

Output Should Be:

Mine can past your testcase with no problem, but I am still failing that testcase

i also could solve NewAccount's test case but still got batch #5 case #2 wrong. make sure you can solve:

etc.. both dots should be 1, one of them should not be 2.

The test cases for this problem can be found at https://www.cemc.uwaterloo.ca/contests/computing/2019/index.html

I don't think the DMOJ judge is reflective of the CCC Grader. My python code works fine on CCC Grader but TLEs on DMOJ.

https://dmoj.ca/submission/1214670

https://gyazo.com/2035b158e7266b140435bc1d188128a7

Although this is the case, if your approach is correct, you can certainly solve this problem on DMOJ.

Try submitting with PyPy

The CCC grader uses PYPY instead of CPython.

Can a conveyor lead to a wall?

Indeed.

Any way to make java code faster? TLE on a couple cases with large inputs.

look here under java

There is one scenario where the robot spawns right next to a camera.

I heard that tripped quite a few people on the CCC

I'm getting 10/15 due to Batch #3 saying "clipped". What does this mean? What is the correct answer for the first test case of Batch #3? https://gyazo.com/6224aad6d59e8f8a5fe6e520db78f6f2

When you fail a test case and your program has output (with partial output enabled), you are allowed to see a limited amount of your output. As for what you are getting wrong, I suggest you think about possible edge cases. If you ever need in-depth help, visit the dmoj discord channel at https://discord.com/invite/EgJVpxz.

Can't get partial on this one

Darcy_Liu, YOU'RE UNDER ARREST

It has come to our attention that you've been copying and pasting solutions on DMOJ. This does not show good sportsmanship and is unfair to DMOJistan. Blindly copying and pasting code does not improve your own abilities and breaks DMOJ's rules.

Proof:

Example 1: COCI '14 Contest 7 #3 ACM - https://dmoj.ca/problem/coci14c7p3

Offender's Code: https://dmoj.ca/submission/1046085

Source: https://dmoj.ca/src/1036320

Example 2: CCC '10 S3 - Firehose - https://dmoj.ca/problem/ccc10s3

Offender's Code: https://dmoj.ca/src/1026958

Source: https://dmoj.ca/src/375814

Darcy_Liu is the enemy of the people. He tries to copy code but the state is always watching.

Darcy___Liu has betrayed the revolution.

Edit 1:

Edit 2:

It appears that you have yet to update your history books and databases. CopyPastePolice is an ally to the state and we are preparing an invasion of PETHCS.

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you have to have solved the problems first to view the source links

How does CopyPastePolice view other people's submissions with only 1 problem solved?

cuz ur mom lol

gottem

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