Code Golf: Mix the nuts so that none of the same kind are touching

Question

Input:

Input is a randomized array of nuts (in your language), the possible nuts follow. Your program must have a way of representing each kind of nut, such as an integer code. Program must be able to handle any size array of any configuration of nuts.

Possible Nuts:

Kola nut
Macadamia
Mamoncillo
Maya nut
Mongongo
Oak acorns
Ogbono nut
Paradise nut
Pili nut
Pistachio
Walnut

Output:

Output must be the array sorted in such a fashion that there are no adjacent nuts of the same kind. If this is impossible, the output should be an empty array.

Example Input (simplified):

["walnut", "walnut", "pistachio"]

Example Output:

["walnut", "pistachio", "walnut"]

Solutions may not simply shuffle the array until it becomes unique by chance. The sort employed must be a deterministic one

Answer 1

GolfScript, 42 41 37 38 characters

~.`{\`{=}+%1-,}+$.,)2//zip[]*.2<..&=*p

The code expects input on STDIN and prints result to STDOUT, e.g.:

> ["walnut" "walnut" "walnut" "macadamia" "pistachio"]
["walnut" "macadamia" "walnut" "pistachio" "walnut"]

> ["walnut" "walnut" "walnut" "macadamia" "walnut"]
[]

The script became longer than expected but I suppose there is room for improvement.

Edit: The case of a list with a single item costs me 1 character (the best comparison I could come up with is the same as Peter's).

Answer 2

Brachylog v2, 10 bytes

p.¬{s₂=}∨Ė

Try it online!

Brute-force solution. (This is a function, allowed because the challenge does not say "full program".) It's also mostly a direct translation of the spec (the only real subtlety is that I managed to arrange things so that all the implicit constraints arrived in exactly the right places, thus not needing any extra characters to disambiguate them).

Note that this is a generic algorithm for rearranging any sort of list so that it does not have two touching elements; it can handle string representations of the elements, and it can handle integer codes just as well. So it doesn't really matter how the "Your program must have a way of representing each kind of nut, such as an integer code." requirement from the question is interpreted.

Explanation

p.¬{s₂=}∨Ė
p            Find a permutation of {the input}
  ¬{   }     which does not have the following property:
    s₂         it contains a pair of adjacent elements
      =        that are equal
        ∨    {no constraint on what value the equal elements can have}
 .           If you find such a permutation, output it.
        ∨    If no permutation is found, ignore the input and
         Ė     {output} an empty list

Answer 3

GolfScript, 32 chars

~:x{]x\-,}$.,)2//zip[]*.2<..&=*`

Same input and output format as Howard's solution.

Answer 4

J, 80 characters

]`_:@.(0<2&([:+/=/\))({.~-:@#),((],.|.)~>.@-:@#)<"1;(\:#&.>)(</.])[;.1' ',1!:1[1

Not really in the same league as Golfscript on this one. I suspect there are gains to be made, but the 14 characters needed just to get the list into the program [;.1' ',1!:1[1 is a major handicap.

Basically the program takes in the list, groups similar items together, sorts by number of items in each group descending, and alternates the output between the first half and the second half of the list. The rest if the code gets rid of extraneous items and decides if the list is valid output (outputting infinity _ if it isn't).

Example:

macadamia walnut walnut pistachio walnut

group (</.]):

macadamia walnut walnut walnut pistachio

sort (\:#&.>):

walnut walnut walnut macadamia pistachio

ravel ((],.|.)~>.@-:@#):

walnut macadamia walnut pistachio walnut

Answer 5

Jelly, 14 bytes

Ġz0UẎḟ0ịµẋ⁻ƝẠ$

Try it online!

The last 6 bytes can be removed if we can have undefined behavior for invalid inputs.

Answer 6

Stax, 10 bytes

│éÿ∞å[zàL⌂

Run and debug it

Here's the same program unpacked, ungolfed, and commented.

|T      get all permutations
{       block to filter by
  :g_=  after dropping repeated elements, it's still equal
f       execute filter
|c      terminate and pop if falsy (no match)
hJ      take the first permutation, and join with spaces

Run this one