This function retrieves the parent node id of a given node from an adjacency list of a tree.
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.
A integer of length 1 should be returned for all nodes except the root. For the latter, the function will return NULL.