(更新:刚刚贴错题目)
证明NPC:
Given an integer k,
a universe U and a family S = {S1, S2, . . . , Sm}
whose union equals U, where each Si is a subset of U,
find out whether there is a sub-family C ⊆ S of at most size k
whose union is the universe U.
请各位大大给个指点
我目前完全没想法
想不到要Reduction到什么东西QQ