unvised是个set，为啥这么写会寄，机房大佬讲的听不懂

for(auto v : unvised) {
                    int x = v;
                    if(G[u].count(v)) continue;
                    vis[x] = true;
                    q.push(x);
                    fa[v] = find(u);
                    unvised.erase(v);
                    rest --;
                }