给邻接表的图实现添加一个边是否存在的判定

[Rickxiaoxin]

在rust的邻接表实现图的程序中，对于添加边（或删除边）缺少了确认边存在（或不存在）。这会导致如果出现边存在的情况下添加边导致列表中存在重复的边。

添加一个判定后可避免该结果的产生

[krahets]

Thanks for the PR! In this graph, we do not limit the number of edges between two vertices.

[Rickxiaoxin]

Thank you very much for reading and responding. This is my first PR, and it may not fully comply with the process. I apologize for any inconvenience this may have caused you！