构造一个集合，里面的数都是丑数，那么任意取一个集合内的数，假设为k，那么2k,3k,5k,7k也是丑数，丢进集合中。因为要求第n小，那么考虑优先队列取前n小数，从1开始扩展集合，重复n次操作，最后的数便是答案。(Construct a set whose numbers are all ugly numbers, then take the number of a collection arbitrarily, assuming that it is K. Then 2K, 3k, 5K and 7K are also ugly numbers and are thrown into the set. Since the request for n is small, then the first N decimal number of priority queues is considered, and the set is extended from 1, and the N operation is repeated, and the final number is the answer.)