<s>
In	O
computer	B-General_Concept
science	I-General_Concept
and	O
artificial	B-Application
intelligence	I-Application
,	O
combinatorial	B-Algorithm
search	I-Algorithm
studies	O
search	B-Application
algorithms	I-Application
for	O
solving	O
instances	O
of	O
problems	O
that	O
are	O
believed	O
to	O
be	O
hard	O
in	O
general	O
,	O
by	O
efficiently	O
exploring	O
the	O
usually	O
large	O
solution	O
space	O
of	O
these	O
instances	O
.	O
</s>
<s>
Combinatorial	B-Algorithm
search	I-Algorithm
algorithms	O
achieve	O
this	O
efficiency	O
by	O
reducing	O
the	O
effective	O
size	O
of	O
the	O
search	O
space	O
or	O
employing	O
heuristics	O
.	O
</s>
<s>
Classic	O
combinatorial	B-Algorithm
search	I-Algorithm
problems	O
include	O
solving	O
the	O
eight	O
queens	O
puzzle	O
or	O
evaluating	O
moves	O
in	O
games	O
with	O
a	O
large	O
game	O
tree	O
,	O
such	O
as	O
reversi	O
or	O
chess	B-Application
.	O
</s>
<s>
A	O
study	O
of	O
computational	O
complexity	O
theory	O
helps	O
to	O
motivate	O
combinatorial	B-Algorithm
search	I-Algorithm
.	O
</s>
<s>
Combinatorial	B-Algorithm
search	I-Algorithm
algorithms	O
are	O
typically	O
concerned	O
with	O
problems	O
that	O
are	O
NP-hard	O
.	O
</s>
<s>
Common	O
algorithms	O
for	O
solving	O
combinatorial	B-Algorithm
search	I-Algorithm
problems	O
include	O
:	O
</s>
<s>
Lookahead	O
is	O
an	O
important	O
component	O
of	O
combinatorial	B-Algorithm
search	I-Algorithm
,	O
which	O
specifies	O
,	O
roughly	O
,	O
how	O
deeply	O
the	O
graph	B-Application
representing	O
the	O
problem	O
is	O
explored	O
.	O
</s>
<s>
The	O
need	O
for	O
a	O
specific	O
limit	O
on	O
lookahead	O
comes	O
from	O
the	O
large	O
problem	O
graphs	O
in	O
many	O
applications	O
,	O
such	O
as	O
computer	B-Application
chess	I-Application
and	O
computer	B-Application
Go	I-Application
.	O
</s>
<s>
A	O
naive	O
breadth-first	B-Algorithm
search	I-Algorithm
of	O
these	O
graphs	O
would	O
quickly	O
consume	O
all	O
the	O
memory	O
of	O
any	O
modern	O
computer	O
.	O
</s>
<s>
More	O
sophisticated	O
search	O
techniques	O
such	O
as	O
alpha-beta	B-Algorithm
pruning	I-Algorithm
are	O
able	O
to	O
eliminate	O
entire	O
subtrees	O
of	O
the	O
search	O
tree	O
from	O
consideration	O
.	O
</s>
