apriori算法，逐层搜索的迭代方法，首先寻找1-项频繁集的集合，集合记做L1， L1用于寻找两项频繁集合L2，L2用于寻找L3，如此下去，直到不能找K项频繁集合-apriori algorithm，Layer by layer search iterative method, first of all, to find a set of 1- frequent itemsets, set to remember to do L1, L1 used to find two frequent s