Distance in Tree
题目描述
A tree is a connected graph that doesn't contain any cycles.
The distance between two vertices of a tree is the length (in edges) of the shortest path between these vertices.
You are given a tree with n vertices and a positive number k. Find the number of distinct pairs of the vertices which have a distance of exactly k between them. Note that pairs (v, u) and (u, v) are considered to be the same pair.
The first line contains two integers n and k (1≤n≤50000, 1≤k≤500) − the number of vertices and the required distance between the vertices.
Next n-1 lines describe the edges as "ai bi" (without the quotes) (1≤ai,bi≤n, ai≠bi), where ai and bi are the vertices connected by the i-th edge. All given edges are different.
Print a single integer − the number of distinct pairs of the tree's vertices which have a distance of exactly k between them.
Please do not use the %lld specifier to read or write 64-bit integers in C++. It is preferred to use the cin, cout streams or the %I64d specifier.
5 2
1 2
2 3
3 4
2 5
4
5 3
1 2
2 3
3 4
4 5
2
In the first sample the pairs of vertexes at distance 2 from each other are (1, 3), (1, 5), (3, 5) and (2, 4).
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