“…It is clear that the longest path problem is NP-hard on every class of graphs on which the Hamiltonian path problem is NP-complete; note that, the Hamiltonian path problem is known to be NP-complete on general graphs [12,13], and remains NP-complete even when restricted to some small classes of graphs such as split graphs [15], chordal bipartite graphs, split strongly chordal graphs [19], directed path graphs [20], circle graphs [7], planar graphs [13], and grid graphs [16]. On the other hand, there are several classes of graphs on which the Hamiltonian path problem admits polynomial time solutions; these classes include proper interval graphs [3], interval graphs [1,5,8], circular-arc graphs [8], biconvex graphs [2], and cocomparability graphs [6].…”