Can you solve the river crossing riddle? - Lisa Winer

翻译人员: Kai Ma 校对人员: Marini Qian

As a wildfire rages through the grasslands,

一场大火席卷了草原

three lions and three wildebeest flee for their lives.

三头狮子和三只牛羚正在逃命

To escape the inferno,

为了逃离这场大火

they must cross over to the left bank of a crocodile-infested river.

他们必须渡过一条潜伏着鳄鱼的小河

Fortunately, there happens to be a raft nearby.

幸运的是，附近恰好有一个小木筏

It can carry up to two animals at a time,

一次可供最多两只动物渡河

and needs as least one lion or wildebeest on board

但每次至少有一只动物

to row it across the river.

控制小木筏渡河

There's just one problem.

只有一个问题

If the lions ever outnumber the wildebeest on either side of the river,

一旦河的任何一边 狮子的数量多于牛羚

even for a moment,

即使只是很短的时间

their instincts will kick in, and the results won't be pretty.

狮子也会因本能而攻击牛羚

That includes the animals in the boat when it's on a given side of the river.

小木筏上的动物将会同时算在河的两边

What's the fastest way for all six animals to get across

那么 这六只动物怎样才能最快地渡河？

without the lions stopping for dinner?

并且牛羚不会被狮子所攻击

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

（可以暂停视频 思考一下这个问题）

Answer in: 3

（3秒后公布答案）

Answer in: 2

（2秒后公布答案）

Answer in: 1

（1秒后公布答案）

If you feel stuck on a problem like this,

如果你认为毫无头绪

try listing all the decisions you can make at each point,

可以试着在纸上列举出 所有时刻的每一种的可能性

and the consequences each choice leads to.

以及它们所导致的后果

For instance, there are five options for who goes across first:

比如 第一次渡河有五种选择

one wildebeest,

一只牛羚

one lion,

一头狮子

two wildebeest,

两只牛羚

two lions,

两头狮子

or one of each.

或者每种一只

If one animal goes alone,

如果第一次让一只动物渡河

it'll just have to come straight back.

那么它只能原路返回

And if two wildebeest cross first,

而如果让两只牛羚渡河

the remaining one will immediately get eaten.

那么剩下的一只牛羚就会立刻被吃掉

So those options are all out.

所以现在只剩下两种可能

Sending two lions,

要么先让两头狮子渡河

or one of each animal,

要么让一头狮子和一只牛羚渡河

can actually both lead to solutions in the same number of moves.

这两种选择后 各自又产生了五种可能性

For the sake of time, we'll focus on the second one.

由于时间的关系 我们直接看第二种

One of each animal crosses.

先让每种动物各一只渡河

Now, if the wildebeest stays and the lion returns,

然后 如果让牛羚待在对岸 让狮子返回

there will be three lions on the right bank.

那么一边就会有三头狮子

Bad news for the two remaining wildebeest.

剩下的两只牛羚就性命难保了

So we need to have the lion stay on the left bank

所以我们需要让狮子待在对岸

and the wildebeest go back to the right.

让牛羚返回

Now we have the same five options,

现在我们又有同样的五种选择

but with one lion already on the left bank.

但是既然对岸已经有一头狮子

If two wildebeest go, the one that stays will get eaten,

如果同时让两只牛羚渡河 剩下的一只就会被吃掉

and if one of each animal goes,

如果每种动物各取一只

the wildebeest on the raft will be outnumbered

那么小木筏上的牛羚

as soon as it reaches the other side.

在到达对岸时就会被吃掉

So that's a dead end,

这两种方法又被排除掉了

which means that at the third crossing,

也就是说 第三次渡河

only the two lions can go.

只能让两只狮子同时渡河

One gets dropped off,

然后在对岸放下一只

leaving two lions on the left bank.

对岸就有两头狮子

The third lion takes the raft back to the right bank

第三头狮子独自返回

where the wildebeest are waiting.

原来的一边还是有三只牛羚

What now?

现在怎么办？

Well, since we've got two lions waiting on the left bank,

既然我们已经让两头狮子到达了河对岸

the only option is for two wildebeest to cross.

唯一的选择就是让两只牛羚过去

Next, there's no sense in two wildebeest going back,

然后 由于让这两只牛羚直接返回是毫无意义的

since that just reverses the last step.

也就是重复了上一步

And if two lions go back,

而如果让两只狮子同时返回

they'll outnumber the wildebeest on the right bank.

又会导致狮子的数量多于牛羚

So one lion and one wildebeest take the raft back

所以只能各取一只返回

leaving us with one of each animal on the left bank

河对岸每种动物各有一只

and two of each on the right.

原来的一边每种动物各有两只

Again, there's no point in sending the lion-wildebeest pair back,

同理 让每种动物各一只再次渡河又会重复上一步骤

so the next trip should be either a pair of lions

所以现在只能让两头狮子渡河

or a pair of wildebeest.

或者让两只牛羚渡河

If the lions go, they'd eat the wildebeest on the left, so they stay,

如果让两头狮子渡河 他们会立即吃掉对岸的牛羚

and the two wildebeest cross instead.

所以只能让两只牛羚渡河

Now we're quite close because the wildebeest are all where they need to be

现在 我们已经胜利在望了

with safety in numbers.

因为三只牛羚都到达了河对岸

All that's left is for that one lion to raft back

只需要再让剩下的一头狮子上船

and bring his fellow lions over one by one.

然后每次带着一只狮子渡河即可

That makes eleven trips total,

这样一共需要十一步

the smallest number needed to get everyone across safely.

也就是所有动物渡河所需的最少步骤

The solution that involves sending both lions on the first step works similarly,

如果第一步将两头狮子送往河对岸 最后也能以类似的方式解决这一问题

and also takes eleven crossings.

同样需要十一步

The six animals escape unharmed from the fire just in time

六只动物及时逃离了危险

and begin their new lives across the river.

并在河的对岸开始了新的生活

Of course, now that the danger's passed,

当然 既然危险已经过去

it remains to be seen how long their unlikely alliance will last.

他们的合作关系还能否继续呢？