Walkthrough - Example Tree

Consider a tree rooted at $3$ with left child $5$ and right child $1$. Node $5$ has children $6$ and $2$. Node $1$ has children $0$ and $8$. Find LCA($5$, $1$). Call on root $3$. Left subtree returns $5$ (found target). Right subtree returns $1$ (found target). Both are non-null, so $3$ is the LCA.

Now find LCA($5$, $4$) where $4$ is a child of $2$. Call on root $3$. Left subtree finds $5$ first and returns it. But $4$ is also in the left subtree under $2$. The left call returns $5$ (since $5$ is encountered before reaching $4$'s subtree). Right returns null. So $5$ is the LCA. Node $5$ is an ancestor of $4$, confirming the answer.