If your answer is too large, you are double-counting paths. Make sure to subtract same-subtree pairs properly. If it is too small, you might be skipping paths that cross multiple centroids or marking nodes as removed incorrectly. Print the decomposition tree to verify structure. Each node should appear exactly once.
Check that subtree sizes halve at each level and that no component exceeds after removing a centroid. If the code times out, you are recomputing sizes incorrectly or not marking nodes as removed efficiently. Use a boolean array for removal checks.
Space complexity is for the data structures used.