Seating people in a movie theater

Question

This is based on an article I read about puzzles and interview questions asked by large software companies, but it has a twist...

General question:

What is an algorithm to seat people in a movie theater so that they sit directly beside their friends but not beside their enemies.

Technical question:

Given an N by M grid, fill the grid with N * M - 1 items. Each item has an association Boolean value for each of the other N * M - 2 items. In each row of N, items directly beside other items should have a positive association value for the other. Columns however do not matter, i.e. an item can be "enemies" with the item in front of it. Note: If item A has a positive association value for B, then that means B also has a positive association value for A. It works the same for negative association values. An item is guarenteed to have a positive association with atleast one other item. Also, you have access to all of the items and their association values before you start placing them in the grid.

Comments:

I have been researching this problem and thinking about it since yesterday, from what I have found it reminds me of the bin packing problem with some added requirements. In some free time I attempted to implement it, but large groups of "enemies" were sitting next to each other. I am sure that most situations will have to have atleast one pair of enemies sitting next to each other, but my solution was far from optimal. It actually looked as if I had just randomized it.

As far as my implementation went, I made N = 10, M = 10, the number of items = 99, and had an array of size 99 for EACH item that had a randomized Boolean value that referred to the friendship of the corresponding item number. This means that each item had a friendship value that corresponded with their self as well, I just ignored that value.

I plan on trying to reimplement this again later and I will post the code. Can anyone figure out a "good" way to do this to minimize seating clashes between enemies?

Answer 1

This problem is NP-Hard.
Define L={(G,n,m)|there is a legal seating for G in m×m matrix,(u,v) in E if u is friend of v} L is a formal definition of this problem as a language.

Proof:
We will show Hamiltonian-Problem ≤ (p) 2-path ≤ (p) This-problem in 2 steps [Hamiltonian and 2-path defined below], and thus we conclude this problem is NP-Hard.

(1) We will show that finding two paths covering all vertices without using any vertex twice is NP-Hard [let's call such a path: 2-path and this problem as 2-path problem]
A reduction from Hamiltonian Path problem:

input: a graph G=(V,E)
Output: a graph G'=(V',E) where V' = V U {u₀}.

Correctness:

• if G has Hamiltonian Path: v₁→v₂→...→vn, then G' has 2-path: v₁→v₂→...→vn,u₀
• if G' has 2-path, since u₀ is isolated from the rest vertices, there is a path: v₁→...→vn, which is Hamiltonian in G.

Thus: G has Hamiltonian path 1 ⇔ G' has 2-path, and thus: 2-path problem is NP-Hard.

(2)We will now show that our problem [L] is also NP-Hard:
We will show a reduction from the 2-path problem, defined above.

input: a graph G=(V,E)
output: (G,|V|+1,1) [a long row with |V|+1 sits].

Correctness:

• If G has 2-path, then we can seat the people, and use the 1 sit gap to use as a 'buffer' between the two paths, this will be a legal perfect seating since if v₁ is sitting next to v₂, then v₁ v₁→v₂ is in the path, and thus (v₁,v₂) is in E, so v₁,v₂ are friends.
• If (G,|V|+1,1) is legal seat:[v₁,...,vk,buffer,vk+1,...,vn] , there is a 2-path in G, v₁→...→vk, vk+1→...→vn

Conclusion: This problem is NP-Hard, so there is not known polynomial solution for it.

Exponential solution: You might want to use backtracking solution: which is basically: create all subsets of E with size |V|-2 or less, check which is best.

static best <- infinity
least_enemies(G,used):
  if |used| <= |V|-2:
     val <- evaluate(used)
     best <- min(best,val)
  if |used| == |V|-2: 
     return
  for each edge e in E-used: //E without used 
     least_enemies(G,used + e)

in here we assume evaluate(used) gives the 'score' for this solution. if this solution is completely illegal [i.e. a vertex appear twice], evaluate(used)=infinity. an optimization can of course be made, trimming these cases. to get the actual sitting we can store the currently best solution.

(*)There are probably better solutions, this is just a simple possible solution to start with, the main aim in this answer is proving this problem is NP-Hard.

EDIT: simpler solution:
Create a graph G'=(V U { u₀ } ,E U {(u₀,v),(v,u₀) | for each v in V}) [u₀ is a junk vertex for the buffer] and a weight function for edges:

w((u,v)) = 1    u is friend of v
w((u,v)) = 2    u is an enemy v
w((u0,v)) = w ((v,u0)) = 0

Now you got yourself a classic TSP, which can be solved in O(|V|^2 * 2^|V|) using dynamic programming.

Note that this solution [using TSP] is for one lined theatre, but it might be a good lead to find a solution for the general case.