Recognizing HH-free, HHD-free, and Welsh-Powell Opposition Graphs

In this paper, we consider the recognition problem on three classes of perfectly orderable graphs, namely, the HH-free, the HHD-free, and the Welsh-Powell opposition graphs (or WPO-graphs). In particular, we prove properties of the chordal completion of a graph and show that a modified version of the classic linear-time algorithm for testing for a perfect elimination ordering can be efficiently used to determine in O(n min {m α(n,n), m + n 2 log n}) time whether a given graph G on n vertices and m edges contains a house or a hole; this implies an O(n min {m α(n,n), m + n 2 log n})-time and O(n+m)-space algorithm for recognizing HH-free graphs, and in turn leads to an HHD-free graph recognition algorithm exhibiting the same time and space complexity. We also show that determining whether the complement G of the graph G is HH-free can be efficiently resolved in O(n m) time using O(n 2) space, which leads to an O(n m)-time and O(n 2)-space algorithm for recognizing WPO-graphs. The previously best algorithms for recognizing HH-free, HHD-free, and WPO-graphs required O(n 3) time and O(n 2) space.