设集合S={x1,x2,…,xn}是一个正整数集合，c是一个正整数，子集和问题判定是否存在S的一个子集S1，使S1中的元素之和为c。试设计一个解子集和问题的回溯法。-Let the set S = {x1, x2, ..., xn} is a set of positive integers, c is a positive integer, a subset of the problem and determine whether there is a subset S S1, so that the elements of S1 and is c. Try to design a solution subset and problems backtracking.