Edge Cases (Small trees)

If $n=1$, the tree is a single node with no edges. The answer for that node is $0$ because the sum of distances to itself is $0$. No DFS needed, return $0$. If $n=2$, the tree is a single edge connecting two nodes. Answer for each node is $1$ because the distance to the other node is $1$.

Down pass computes this immediately. Always test these small cases. They often reveal off-by-one errors in your recurrence formula, especially in the up[v] calculation where you add or subtract sizes.