F - Close Group Editorial by en_translator

Let \(V\) be the vertex set of the graph, \(S\) be a non-empty subset of \(V\) and \(f(S)\) be the answer for this problem when the input is the subgraph induced by \(S\).
What we want is \(f(V)\), but it is necessary to calculate all \(f(S)\) when obtaining \(f(V)\), so let us consider calculating all \(f(S)\).

If the induced subgraph by \(S\) is complete, then \(f(S) = 1\); otherwise \(f(S)\) is determined by \(\displaystyle \min_{T \subset S}f(T)+f(S \setminus T)\).
Therefore, one can determine \(f(S)\) in order that smaller subsets are calculated earlier.

Let us consider the complexity.
One can determine if each induced subgraph is complete in \(O(2^N N^2)\) time for all \(S\).
The problem is \(\displaystyle \min_{T \subset S}f(T)+f(S \setminus T)\) part.
If \(T\) is iterated through all subset of \(V\), the total computation time for all \(S\) will be \(O(2^N \cdot 2^N)=O(4^N)\); actually, however, \(S\) can be processed in a total of \(O(3^N)\) time, since \(T\) can only be a subset of \(S\), and

\[ \sum_{S \subset V} 2^{|S|}=3^N. \]

This formula can also be understood from the fact that each element of \(V\) can be either

  • contained in both \(S\) and \(T\),
  • contained only in \(S\), or
  • not contained in either set,


Sample Code (C++) : https://atcoder.jp/contests/abc187/submissions/19103570
Sample Code (Python) : https://atcoder.jp/contests/abc187/submissions/19103577

The answer for this problem is the chromatic number of complement graph when

  • two vertices directly connected by an edge can be painted with the same color (if not, the edge is removed), and
  • two vertices not directly connected by edges cannot be painted with the same color.

It is known that the chromatic number of a graph calculated in a total of \(O(2^NN)\) time, so the original problem can be solved in a total of \(O(2^NN)\) time as well.

Related Materials (Japanese): https://www.slideshare.net/wata_orz/ss-12131479

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