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• Your research team has found a prehistoric virus preserved in the permafrost

• and isolated it for study.

• After a late night working,

• you're just closing up the lab when a sudden earthquake hits

• and knocks out the power.

• As the emergency generators kick in, an alarm confirms your worst fears:

• all the sample vials have broken.

• The virus is contained for now,

• but unless you can destroy it,

• the vents will soon open and unleash a deadly airborne plague.

• Without hesitation, you grab your HazMat suit

• and get ready to save the world.

• The lab is a four by four compound of 16 rooms

• with an entrance on the northwest corner and an exit at the southeast.

• Each room is connected to the adjacent ones by an airlock,

• and the virus has been released in every room except the entrance.

• To destroy it, you must enter each contaminated room

• and pull its emergency self-destruct switch.

• But there's a catch.

• Because the security system is on lockdown,

• once you enter the contaminated room,

• you can't exit without activating the switch,

• and once you've done so,

• you won't be able to go back in to that room.

• You start to draw out possible routes on a pad of paper,

• but nothing seems to get you to the exit without missing at least one room.

• So how can you destroy the virus in every contaminated room

• and survive to tell the story?

• Pause here if you want to figure it out for yourself.

• Answer in: 3

• Answer in: 2

• Answer in: 1

• If your first instinct is to try to graph your possible moves on a grid,

• you've got the right idea.

• This puzzle is related to the Hamiltonian path problem

• named after the 19th century Irish mathematician William Rowan Hamilton.

• The challenge of the path problem

• is to find whether a given graph has a Hamiltonian path.

• That's a route that visits every point within it exactly once.

• This type of problem, classified as NP-complete,

• is notoriously difficult when the graph is sufficiently large.

• Although any proposed solution can be easily verified,

• we have no reliable formula or shortcut for finding one,

• or determining that one exists.

• And we're not even sure if it's possible for computers

• to reliably find such solutions, either.

• This puzzle adds a twist to the Hamiltonian path problem

• in that you have to start and end at specific points.

• But before you waste a ton of graph paper,

• you should know that a true Hamiltonian path

• isn't possible with these end points.

• That's because the rooms form a grid with an even number of rooms on each side.

• In any grid with that configuration,

• a Hamiltonian path that starts and ends in opposite corners is impossible.

• Here's one way of understanding why.

• Consider a checkerboard grid with an even number of squares on each side.

• Every path through it will alternate black and white.

• These grids will all also have an even total number of squares

• because an even number times and even number is even.

• So a Hamiltonian path on an even-sided grid that starts on black

• will have to end on white.

• And one that starts on white will have to end on black.

• However, in any grid with even numbered sides,

• opposite corners are the same color,

• so it's impossible to start and end a Hamiltonian path on opposite corners.

• It seems like you're out of luck,

• unless you look at the rules carefully and notice an important exception.

• It's true that once you activate the switch in a contaminated room,

• it's destroyed and you can never go back.

• But there's one room that wasn't contaminated - the entrance.

• This means that you can leave it once without pulling the switch

• and return there when you've destroyed either of these two rooms.

• The corner room may have been contaminated from the airlock opening,

• but that's okay because you can destroy the entrance after your second visit.

• That return trip gives you four options for a successful route,

• and a similar set of options if you destroyed this room first.

• Congratulations. You've prevented an epidemic of apocalyptic proportions,

• but after such a stressful episode, you need a break.

• Maybe you should take up that recent job offer to become a traveling salesman.

Your research team has found a prehistoric virus preserved in the permafrost

B1 中級 新型コロナウイルス 新型肺炎 COVID-19

# TED-ED】ウイルスの謎が解ける？- リサ・ワイナー (【TED-Ed】Can you solve the virus riddle? - Lisa Winer)

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Steven Wu に公開 2021 年 01 月 14 日