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Use these hints if you're stuck. Try solving on your own first.
Find the connected components of the graph. To find connected components, you can use Union Find (Disjoint Sets), BFS, or DFS.
For a node u, the number of nodes that are unreachable from u is the number of nodes that are not in the same connected component as u.
The number of unreachable nodes from node u will be the same for the number of nodes that are unreachable from node v if nodes u and v belong to the same connected component.