def get_steiner(self, query_elements, lefts):
"""
dynamic programming for steiner tree problem
"""
subpaths, root = steiner(self.undirected_graph._graph, query_elements, \
self.undirected_graph.d, \
self.undirected_graph.PT)
n = len(self.undirected_graph._graph)
lk_set = set([])
for lk in subpaths:
ll, rr = lk
lk_set.add(lk)
if (rr, ll) in lk_set:
lk_set.remove(lk)
sub_span = [[] for _ in self.undirected_graph._graph ]
for i, j in lk_set:
sub_span[i].append((j, 0))
sub_span[j].append((i, 0))
subtree = RootedWGraph(sub_span, root)
root = subtree.root_index
if root in lefts:
_LOGGER.debug("swith_root")
subtree.switch_root(lefts)
return subtree