选择一个 $N$ 个点，$N$ 条边的无向联通图 $G$，要求无重边，但可以有自环。

Provide an undirected connected graph $G$ with $N$ vertices and $M$ edges. Here, $G$ may not contain self-loops, but may contain multi-edges.

改为：选择一个 $N$ 个点，$M$ 条边的无向联通图 $G$，要求无自环，但可以有重边。