Middle of a singly linked list of unknown length

[Himanshu Chauhan]

Hi!

I was wondering, In the first parse of a singly linked list of unknown
length, is it possible to know when we are at middle of the linked list?

Regards
--Himanshu

[Richard Tobin]

In article <g2**********@a ioe.org>,
Himanshu Chauhan <ch*********@gm ail.comwrote:

>I was wondering, In the first parse of a singly linked list of unknown
length, is it possible to know when we are at middle of the linked list?

Obviously not, since nothing would be different if extra elements were
added to the end.

-- Richard
--
In the selection of the two characters immediately succeeding the numeral 9,
consideration shall be given to their replacement by the graphics 10 and 11 to
facilitate the adoption of the code in the sterling monetary area. (X3.4-1963)

[Dan]

I was wondering, In the first parse of a singly linked list of unknown
length, is it possible to know when we are at middle of the linked list?

You could do it if you kept track of how many were in the list when you
built it.

[rahul]

On Jun 6, 1:48 pm, Himanshu Chauhan <chauhan....@gm ail.comwrote:

Hi!

I was wondering, In the first parse of a singly linked list of unknown
length, is it possible to know when we are at middle of the linked list?

Regards
--Himanshu

You can keep the count of nodes inserted as a static or global
variable. By counting
the number of iterations, you can decide if you are in the middle of
the list.

Just out of curiosity, this question is meant for some real code or is
it just like that.

[Jens Thoms Toerring]

Himanshu Chauhan <ch*********@gm ail.comwrote:

Hi!

I was wondering, In the first parse of a singly linked list of unknown
length, is it possible to know when we are at middle of the linked list?

Are you looking for the old trick of using two pointers, the first
one always getting set to the next element, the other one being set
to the successor of the next element so, when the second pointer
reaches the end of the list, the first one points to the middle
element? But I don't know if that is allowed by what you call
"first parse".

BTW, this hasn't anything to do with C, so such a question is
much better suited for comp.programmin g.

Regards, Jens
--
\ Jens Thoms Toerring ___ [e-mail]
\______________ ____________ http://toerring.de

[CBFalconer]

Himanshu Chauhan wrote:

>
I was wondering, In the first parse of a singly linked list of
unknown length, is it possible to know when we are at middle of
the linked list?

Yes. Just build the list in two halves, and join them when
building is done. The head of the second half will be the
midpoint, withing an error of one.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.
** Posted from http://www.teranews.com **

[Kenneth Brody]

Himanshu Chauhan wrote:

>
Hi!

I was wondering, In the first parse of a singly linked list of unknown
length, is it possible to know when we are at middle of the linked list?

The me rephrase the problem, and see if you can find a solution:

Drive down this road and stop halfway to a destination which I
have not yet revealed.

--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer .h|
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <mailto:Th***** ********@gmail. com>

[Bartc]

"Kenneth Brody" <ke******@spamc op.netwrote in message
news:48******** *******@spamcop .net...

Himanshu Chauhan wrote:

>>
Hi!

I was wondering, In the first parse of a singly linked list of unknown
length, is it possible to know when we are at middle of the linked list?

The me rephrase the problem, and see if you can find a solution:

Drive down this road and stop halfway to a destination which I
have not yet revealed.

That's not quite the same. In that case you would not know when you got to
the destination so it's unsolveable. A linked list however has a definite
end point, assuming it's not circular.

Better, 'stop halfway to the end of the road of unknown length'. Easily done
by traversing the entire length one and a half times.
--
Bartc

[Richard Tobin]

In article <sl************ *******@nospam. invalid>,
Antoninus Twink <no****@nospam. invalidwrote:

>At least two people have already provided a correct
solution (i.e. run two pointers through the list, one travelling at
half the speed of the other)

That's not a solution to the problem as I interpreted it. It uses
one-and-a-half passes over the list, rather than stopping in the
middle during the first pass.

-- Richard
--
In the selection of the two characters immediately succeeding the numeral 9,
consideration shall be given to their replacement by the graphics 10 and 11 to
facilitate the adoption of the code in the sterling monetary area. (X3.4-1963)

[Eric Sosman]

Antoninus Twink wrote:

On 6 Jun 2008 at 16:36, Keith Thompson wrote:

>CBFalconer <cb********@yah oo.comwrites:

[snip nonsense]

>Right, finding a solution is easy if you're allowed to redefine the
problem.

Vintage CBF. At least two people have already provided a correct
solution (i.e. run two pointers through the list, one travelling at
half the speed of the other),

I think those so-called solutions are ruled out by the
O.P.'s requirement to get the answer "in the first parse"
over the list, which I read as a garbled form of "in the
first pass" over the list. Using two pointers means using
one-and-a-half passes.

Still, I suppose at least he tried, albeit unsuccessfully, to
answer the damn question for once, instead of telling the OP
to get lost and try comp.programmin g.)

Right, that was a mistake on his part. Chuck, I hereby
chastise you for answering an off-topic question instead of
sending the questioner to comp.programmin g where he belongs
and where he'll get better answers.

--
Er*********@sun .com