说明：  遗传算法解决 TSP 问题: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
(Known n the distance between cities, a salesman must visit the n city, and each city can only visit once, and finally must return to the starting city )