Earliest Common Ancestor

Given two nodes $p$ and $q$ in a tree, the earliest common ancestor (ECA) of $p$, $q$ is the earliest node (travelling bottom-up from $p$, $q$) that is an ancestor of both $p$ and $q$. For convenience and to handle degenerate cases (e.g. when $p$, $q$ are the same node or one of them is the root) we let a node be an ancestor of itself. Assume all $n$ nodes in the tree have unique labels labeled fr