1. 城市链表
[问题描述]
　　将若干城市的信息，存入一个带头结点的单链表。结点中的城市信息包括：城市名，城市的位置坐标。要求能够利用城市名和位置坐标进行有关查找、插入、删除、更新等操作。
[基本要求]
　　（1） 给定一个城市名，返回其位置坐标；
　　（2） 给定一个位置坐标P和一个距离D，返回所有与P的距离小于等于D的城市。(1. City chain list
[descr iption of the problem]
The information of a number of cities is deposited into a single chain table with a leading node. The city information in the node consists of the city name and the location of the city. It is required to be able to use the city name and location coordinates for the operation of searching, inserting, deleting, updating and so on.
[basic requirements]
(1) given a city name, return its position coordinates;
(2) given a position coordinate P and a distance D, return all the cities with the distance less than the P equal to the D.)