Hi, Codeforces. I've recently found an interesting problem:
Given an undirected tree with n vertices. Find out how many different ways you can orient the edges of the tree so that the result graph will contain exactly m vertices with zero outdegree modulo 109 + 7, (1 ≤ n ≤ 1000, 0 ≤ m ≤ n).
Could someone help me with this problem and explain the solution?