This function retrieves the parent node id of a given node from an adjacency list of a tree.
get_parent_id2(adj_list, at_node)A integer of length 1 should be returned for all nodes except the root. For the latter, the function will return NULL.
The adjacency list of the tree.
The node whose parent should be extracted.
Remember that the list has to be for a tree, not a general graph. Please see other help pages for more specifications.
This is a low-level function, used within the S4 class TreeHarp. It is not generally meant for use by the user.
If there are nodes that have more than one parent, then a warning is issued.