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Your research team has found a prehistoric virus preserved in the permafrost and isolated it for study.
你的研究团队发现了一种被保存在永冻土层的史前病毒，你们将它分离出来用作研究。
After a late night working, you're just closing up the lab when a sudden earthquake hits and knocks out the power.
在一整晚的工作后，当你正关上实验室的门时，一场突如其来的地震发生了，导致电源断了。
As the emergency generators kick in, an alarm confirms your worst fears: all the sample vials have broken.
启用紧急电源之后，一声警报证实了你最惧怕的事：所有的小药瓶样本已被打碎了。
The virus is contained for now, but unless you can destroy it,
病毒现在还存留着，但是除非你能销毁它，
the vents will soon open and unleash a deadly airborne plague.
不久排风扇就会打开，释放出这种通过空气传播的致命病毒。
Without hesitation, you grab your HazMat suit and get ready to save the world.
毫不犹豫，你抓起了你的防护服准备去拯救世界。
The lab is a four by four compound of 16 rooms with an entrance on the northwest corner and an exit at the southeast.
实验室是由4乘4，16个房间组成的，西北角有一个入口，东南角一个出口。
Each room is connected to the adjacent ones by an airlock,
每个房间都与相邻的房间由气塞连接着，
and the virus has been released in every room except the entrance.
除了入口那间，病毒已经弥漫到了其他所有房间。
To destroy it, you must enter each contaminated room and pull its emergency self-destruct switch. But there's a catch.
想要销毁它，你必须进入每一个被污染的房间，拉下各自的紧急自毁开关。但是这里有个问题。
Because the security system is on lockdown,
因为安全系统是处于一级防范禁闭，
once you enter the contaminated room, you can't exit without activating the switch,
一旦你进入了被污染的房间，你不开启开关就不能逃出去，
and once you've done so, you won't be able to go back in to that room.
一旦你开了开关，你就没法再回到那个房间去。
You start to draw out possible routes on a pad of paper,
你开始在一叠纸上画出可能的路线，
but nothing seems to get you to the exit without missing at least one room.
但是似乎找不到一个能够不错过任何一个房间就能走到出口的路线。
So how can you destroy the virus in every contaminated room and survive to tell the story?
因此，要怎样才能销毁每个被污染的房间，活着逃出来，再给大家说起这段故事呢？
If your first instinct is to try to graph your possible moves on a grid, you've got the right idea.
如果你的直觉是尝试在方格上画出可能的路线的话，你的想法是对的。
This puzzle is related to the Hamiltonian path problem named after the 19th century Irish mathematician William Rowan Hamilton.
这个谜团与汉密尔顿路径有关，它是以19世纪爱尔兰数学家威廉·汉密尔顿命名的。
The challenge of the path problem is to find whether a given graph has a Hamiltonian path.
这个路径问题的挑战是能否在已有图表上找出一条汉密尔顿路径。
That's a route that visits every point within it exactly once.
这是一条要求在每个点上正好划过一次的路线。
This type of problem, classified as NP-complete, is notoriously difficult when the graph is sufficiently large.
这种类型的问题，被归类为NP完全问题，众所周知，当图表足够大时，这问题奇难无比。
Although any proposed solution can be easily verified,
虽然任何提出的解决方法都可以被轻易证实，
we have no reliable formula or shortcut for finding one, or determining that one exists.
我们没有可靠的方程式或捷径来找出这个方法，或证明这个方法存在。
And we're not even sure if it's possible for computers to reliably find such solutions, either.
我们甚至也不确定能否依赖计算机来可靠地找出这种解决方法。
This puzzle adds a twist to the Hamiltonian path problem in that you have to start and end at specific points.
这个难题又将汉密尔顿路径问题提升了一个难度，因为你要在一个特定的点开始，并在另一个特定的点结束。
But before you waste a ton of graph paper,
但是在浪费你无数的稿纸之前，
you should know that a true Hamiltonian path isn't possible with these end points.
你应该知道真正的汉密尔顿路径若用这些开始或结束的点，是无法成立的。
That's because the rooms form a grid with an even number of rooms on each side.
那是因为房间形成一个表格，每边房间的数量都是偶数。
In any grid with that configuration, a Hamiltonian path that starts and ends in opposite corners is impossible.
在任何一个这样结构的表格里，找出一条起点和终点在相反角落的汉密尔顿路径是不可能的。
Here's one way of understanding why. Consider a checkerboard grid with an even number of squares on each side.
这里有一个解释。比如说一个棋盘两边都有一个偶数量的正方格。
Every path through it will alternate black and white.
通过它的每条路径都会黑白交替。
These grids will all also have an even total number of squares because an even number times and even number is even.
这些方格的总数量将是偶数，因为偶数与偶数的积是偶数。
So a Hamiltonian path on an even-sided grid that starts on black will have to end on white.
因此在一个偶数边表格上，汉密尔顿路径若是从黑色方格开始，最终将在白色方格结束。
And one that starts on white will have to end on black.
若是以白色方格开始的话，就会以黑色方格结束。
However, in any grid with even numbered sides, opposite corners are the same color,
但是，在任何偶数边表格中，对角的颜色是相同的，
so it's impossible to start and end a Hamiltonian path on opposite corners.
要找到一个起点和终点在对角的汉密尔顿路径是不可能的。
It seems like you're out of luck, unless you look at the rules carefully and notice an important exception.
似乎你已经没辙了，但是如果你仔细看一下规则，可以注意到一个很重要的例外。
It's true that once you activate the switch in a contaminated room, it's destroyed and you can never go back.
没错，一旦你启动污染房间的开关，它已自毁并且你再也没法回去了。
But there's one room that wasn't contaminated -- the entrance.
但有一个房间是没有被污染的，就是入口的房间。
This means that you can leave it once without pulling the switch
这意味着你可以在不启动开关的情况下离开一次，
and return there when you've destroyed either of these two rooms.
在你销毁与它相邻的两个房间其中之一后，再回到入口房间。
The corner room may have been contaminated from the airlock opening,
角落的房间可能已经因开着的气塞而被污染了，
but that's okay because you can destroy the entrance after your second visit.
但这没问题，因为你可以在第二次进入之后再销毁入口房间。
That return trip gives you four options for a successful route,
重返路线给你提供了四条可行的路线，
and a similar set of options if you destroyed this room first.
如果你先销毁这个房间的话，也会出现类似的方案。
Congratulations. You've prevented an epidemic of apocalyptic proportions,
恭喜，你已经阻止了一个能导致世界末日的传染病，
but after such a stressful episode, you need a break.
渡过这一难关之后，你需要休息一下。
Maybe you should take up that recent job offer to become a traveling salesman.
或许你应该答应那个工作机会，去做一名旅行推销员。
