说明：   给定一张航空图，图中项点代表城市，边代表两城市之间的直通航线。现要求找出一条满足下述限制条件且途经城市最多的旅行路线： （1） 从最西端城市出发，单向从西向东途经若干城市到达最东端城市，然后再从东向西飞回起点（可途经若干个城市） （2） 除起点城市外，任何城市只能访问一次。 对于给定的航空图，请设计一个算法找出一条满足要求的最佳航空路线。
(Given an aviation map, map key points on behalf of the city, while on behalf of direct flights between the two cities. Are now required to find a meet the following restrictions and most of the travel route through the city: (1) starting from the western end of the city, one-way from west to east through several cities in the easternmost city of arrival, and then fly back to the starting point from east to west (can be via a number of cities) (2) In addition to the starting point outside the city, any city can only access once. For a given air map, designed to meet the requirements of an algorithm to find a best air route.)