Algorithm to find lowest common ancestor in directed acyclic graph?

Question

Imagine a directed acyclic graph as follows, where:

• \"A\" is the root (there is always exactly one root)
• each node knows its parent(s)
• the no

Answer 1

Just some wild thinking. What about using both input nodes as roots, and doing two BFS simultaneously step by step. At a certain step, when there are overlapping in their BLACK sets (recording visited nodes), algorithm stops and the overlapped nodes are their LCA(s). In this way, any other common ancestors will have longer distances than what we have discovered.