Question

What is the easiest way to implement lowest common ancestor in Python? I have a tree which is represented by each node having a pointer to its parent, and I want to be able to find the first common ancestor given two nodes. I’ve come up with several ideas but none are particularly appealing

1. Have each node contain a list of its bases, and to perform a join, find the longest common prefix and then take the last element. Unfortunately, I don’t know of any builtin way to do longest common prefix so this requires manual looping.

2. Have each node contain a set of its bases and perform a set intersection, and take the maximum element. But this requires defining custom comparison operators, and I’m not even sure if it would work.

What should I do? I’m looking for something that favors simplicity over performance, so solutions requiring complicated processing are out.

Edit: I discovered that while there isn’t a builtin way, you can do longest common prefix in one line using zip, so it’s still fairly simple.

common = [x for x in zip(*baselists) if len(set(x)) == 1][-1]

Answer 1

Under the assumption that you cannot amend your tree to include depth, you can do the following:

For each Node, recursively traverse the tree upwards until you hit the root. At each parent node, insert the node into a list. This should give you list_a and list_b. Iterate over the shortest list, comparing elements from each list. When you find one that doesn’t match, the previous entry is your largest parent element.