<s>
The	O
bridge	B-Algorithm
and	I-Algorithm
torch	I-Algorithm
problem	I-Algorithm
(	O
also	O
known	O
as	O
The	O
Midnight	O
Train	O
and	O
Dangerous	O
crossing	O
)	O
is	O
a	O
logic	O
puzzle	O
that	O
deals	O
with	O
four	O
people	O
,	O
a	O
bridge	O
and	O
a	O
torch	B-Application
.	O
</s>
<s>
They	O
have	O
one	O
torch	B-Application
and	O
,	O
because	O
it	O
's	O
night	O
,	O
the	O
torch	B-Application
has	O
to	O
be	O
used	O
when	O
crossing	O
the	O
bridge	O
.	O
</s>
<s>
The	O
question	O
is	O
,	O
can	O
they	O
all	O
get	O
across	O
the	O
bridge	O
if	O
the	O
torch	B-Application
lasts	O
only	O
15	O
minutes	O
?	O
</s>
<s>
An	O
obvious	O
first	O
idea	O
is	O
that	O
the	O
cost	O
of	O
returning	O
the	O
torch	B-Application
to	O
the	O
people	O
waiting	O
to	O
cross	O
is	O
an	O
unavoidable	O
expense	O
which	O
should	O
be	O
minimized	O
.	O
</s>
<s>
This	O
strategy	O
makes	O
A	O
the	O
torch	B-Application
bearer	O
,	O
shuttling	O
each	O
person	O
across	O
the	O
bridge	O
:	O
</s>
<s>
But	O
then	O
C	O
or	O
D	O
must	O
cross	O
back	O
to	O
bring	O
the	O
torch	B-Application
to	O
the	O
other	O
side	O
,	O
and	O
so	O
whoever	O
solo-crossed	O
must	O
cross	O
again	O
.	O
</s>
<s>
The	O
torch	B-Application
itself	O
may	O
expire	O
in	O
a	O
short	O
time	O
and	O
so	O
serve	O
as	O
the	O
time	O
limit	O
.	O
</s>
