求解集合覆盖。给定一个实例I = (X , F),X为一个有限集合，F为X的子集的集合，使得X中的每个元素至少属于F中的一个元素，则认为子集F覆盖了X的元素。

集合覆盖问题就是，找出含X的子集个数最少的子集C属于F，使得C覆盖了X中的所有元素。

-For set covering. Given an instance of I = ( X, F ), X is a finite set, F X a subset of the set, so that every element in X at least belong to an F element, that subset F covering X elements.Set cover problem is to find the subset of X, containing a number of at least a subset of C belongs to F, the C covering all elements in the X.