说明：  子集和问题的一个实例（s，t).其中s是一个正整数集合，t是一个正整数。子集和问题判断是否存在s中的一个子集s1，使得s1的和等于t
(And problems in a subset of an instance (s, t). Which is a positive integer s collection, t is a positive integer. Subset and to determine whether problems exist s in a subset of s1, made equal to s1 and t)