Algorithms and hardness results for happy coloring problems

In a vertex-colored graph, an edge is happy if its endpoints have the same color. Similarly, a vertex is happy if all its incident edges are happy. Motivated by the computation of homophily in social networks, we consider the algorithmic aspects of the following Maximum Happy Edges (k-MHE) problem: given a partially k-colored graph G, find an extended full k-coloring of G maximizing the number of happy edges. When we want to maximize the number of happy vertices, the problem is known as Maximum Happy Vertices (k-MHV). We further study the complexity of the problems and their weighted variants. For instance, we prove that for every k >= 3, both problems are NP-complete for bipartite graphs and k-MHV remains hard for split graphs. In terms of exact algorithms, we show both problems can be solved in time O(2^n), and give an even faster O(1.89^n)-time algorithm when k = 3. From a parameterized perspective, we give a linear vertex kernel for Weighted k-MHE, where edges are weighted and the goal is to obtain happy edges of at least a specified total weight. Finally, we prove both problems are solvable in polynomial-time when the graph has bounded treewidth or bounded neighborhood diversity.