说明：  已知n个城市之间的相互距离，现有一个推销员必须遍访这n个城市，并且每个城市只能访问一次，最后又必须返回出发城市。如何安排他对这些城市的访问次序，可使其旅行路线的总长度最短？
(known cities between the distance from the existing one must salesman n visited this city, every city and can only be visited once, finally starting to return to the city. How to arrange for him to visit these cities priorities, it can travel routes, the total length of the shortest?)