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Topic: arrange 0,1 and 2 (Read 1646 times)

rahul_832
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arrange 0,1 and 2
« on: Jan 4^th, 2009, 9:42am »

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Given an array of red, green and blue balls arrange them in groups of all red together, greens together and blue together. Do in a single scan of the array.

This is same as You have an array containing only '0's, '1's and '2's. Club same items together in single scan.

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towr
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Re: arrange 0,1 and 2
« Reply #1 on: Jan 4^th, 2009, 10:51am »

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Here's an older thread on this problem: http://www.ocf.berkeley.edu/~wwu/cgi-bin/yabb/YaBB.cgi?board=riddles_cs; action=display;num=1100111371

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rahul_832
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Re: arrange 0,1 and 2
« Reply #2 on: Jan 4^th, 2009, 10:52am »

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sorry i trying searching ...

but was unable to find

thanks

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towr
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Re: arrange 0,1 and 2
« Reply #3 on: Jan 4^th, 2009, 11:30am »

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It's difficult to find older threads when the details of the problem keep changing, so I certainly won't hold it against you.

It seems to be known as the http://en.wikipedia.org/wiki/Dutch_national_flag_problem and is due to Edgar Dijkstra.

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rahul_832
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Re: arrange 0,1 and 2
« Reply #4 on: Jan 4^th, 2009, 11:38am »

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thanks towr

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howard roark
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Re: arrange 0,1 and 2
« Reply #5 on: Jan 5^th, 2009, 10:30pm »

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@towr
I was just going through the old thread for this problem. I found the solution you posted:

Quote:

l=m=0
r=n-1
while (r > m)
{
  while (a[r]=B)
   r--;
  switch (a[m])
  {
   case R : m++; break;
   case W : swap(l,m), m++, l++; break;
   case B : swap(m,r), r-- break;
  }
}

Can you tell me if this code is stable sort? I think it is not stable.

Is there a stable solution for this problem which satisfies all the requirements?

« Last Edit: Jan 5^th, 2009, 10:54pm by howard roark »

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towr
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Re: arrange 0,1 and 2
« Reply #6 on: Jan 6^th, 2009, 12:24am »

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on Jan 5^th, 2009, 10:30pm, howard roark wrote:

Can you tell me if this code is stable sort? I think it is not stable.

You're right, it isn't.

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Is there a stable solution for this problem which satisfies all the requirements?

I'm not entirely certain at the moment. I know it came up in one of the thread about this problem, but I haven't been able to find it.

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SMQ
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Re: arrange 0,1 and 2
« Reply #7 on: Jan 6^th, 2009, 5:46am »

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Since we were unable to come up with a stable O(n) sort of just two values, I doubt we (the regulars on this forum) are aware of a stable single-pass sort of three values.

--SMQ

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towr
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Re: arrange 0,1 and 2
« Reply #8 on: Jan 6^th, 2009, 6:02am »

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Meh, you never know, perhaps we were smarter 5 or so years ago. Tongue

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