要求有向树Ｔ的k个独立顶点组成的集合Ｆ，使cost(F)=∑min w(x)*d(x,u)的值达到最小。与有向树k中值问题类似，把有向树变转成为与之等价的二叉树，设Ｔ的以顶点x为根的子树T(x),其左、右儿子顶点分别为y和z。(A set F that consists of a k independent vertex to a tree T is required to minimize the value of the cost