说明：  1、问题陈述 给定N个数，和一个数T，判定是否可以从N个数中取出若干个数，使它们的和等于T。输出：YES或者NO。把N个数看成一个集合，问题就是从这个集合中选出一个子集，使这个子集满足和是T 2、问题分析： 枚举+剪枝。
(1, the problem statement given number N, and a number T, determining whether the number N is removed from certain numbers, so that their sum is equal to T. Output: YES or NO. The number N as a set, the question is chosen from the set a subset, so that this subset is satisfied and T 2, problem analysis: Enumeration+ pruning.)