说明：  已知n个物体的重量依次为W1>W2>W3>…….Wn，一个背包可以容纳物体的重量为W,将这些物体放入背包，要求所放置的物体的重量正好为W。
(There are something,the number is n,the weight is w1>w2>w3>...wn,a bag can hold a weight of w,put these things into the bag,is just the weight.)