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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

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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
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