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  2. 2 Additional Online Test (If Required)
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Problems > Tree Paths of Length K > Editorial

Tree Paths of Length K — Solution & Editorial

Back to the Problem

Rooted DP: cnt[u][d] = nodes at depth d within u's subtree. When merging a child, pairs passing through u combine depths summing to K−1; then shift and add the child's counts. (Centroid decomposition scales to N = 105.)

Complexity: O(N·K)

Watch out: Count pairs BEFORE merging a child, or a node pairs with itself's subtree twice.