说明：  贪心算法求背包问题,分别求出了三种标准1. 按效益值由大到小取物品. 2. 按重量值由小到大取物品 3.按比值pi/wi的值由大到小取物品 其中第3种是最优解
(Greedy algorithm for knapsack problem, respectively, obtained three standard 1. Press-effective value of descending and remove things. 2. By weight the value of items taken from small to big 3. According to the ratio of pi/wi check the value of descending The first of these three kinds of goods is the optimal solution)