Partitioning graphs into connected parts

The 2-Disjoint Connected Subgraphs problem asks if a given graph has two vertex-disjoint connected subgraphs containing prespecified sets of vertices. We show that this problem is NP-complete even if one of the sets has cardinality 2. The Longest Path Contractibility problem asks for the largest integer ℓ for which an input graph can be contracted to the path Pℓ on ℓ vertices. We show that the computational complexity of the Longest Path Contractibility problem restricted to Pℓ-free graphs jumps from being polynomially solvable to being NP-hard at ℓ=6, while this jump occurs at ℓ=5 for the 2-Disjoint Connected Subgraphs problem. We also present an exact algorithm that solves the 2-Disjoint Connected Subgraphs problem faster than O∗(2n) for any n-vertex Pℓ-free graph. For ℓ=6, its running time is O∗(1.5790n). We modify this algorithm to solve the Longest Path Contractibility problem for P6-free graphs in O∗(1.5790n) time.