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(Doubt)Cses Fixed-Length Paths I
By Pawan_Lahoti, history, 18 months ago, In English

I know about centroid decomposition but I cannot understand the solution given on USACO guide...Can someone please give a simple explanation to this problem https://cses.fi/problemset/task/2080

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18 months ago, # |
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15 months ago, # |
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We are exploiting the fact that for any two nodes (u,v), the lca in their centroid tree divides it into two disjoint paths.

Thus when we find a centroid, we effectively want to calculate the summation of cnt_i[x] * cnt_j[y] where cnt_i denotes the cnt of depth = x in child i of centroid and x + y == k. Because of the above mentioned fact, we won't get any repetition.

Time complexity: for each layer of centroids , we will do O(N) operations and because the height of centroid tree is limited to log(N), Time Complexity will be O(Nlog(N)).

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