Before revision:  We also discuss the hardness of the problem , achieving a better lower bound than the previous one [ IPL 2019 ] . We believe our simplified method of analysis can be helpful to deal with similar problems on general undirected graphs .
After revision:  We also discuss the hardness of the problem , achieving a better lower bound than the previous one [ 1 ] . We believe our simplified method of analysis can be helpful to deal with similar problems on general undirected graphs .