一种抠环的办法

int fa[maxn];        //dfs时的父亲
int dfn[maxn], idx;  //访问的时间
int loop[maxn], cnt; //环
void get_loop(int u) {
    dfn[u] = ++ idx;
    for (int i = head[u];i;i=ed[i].nxt) {
        int v = ed[i].to;
        if(v == fa[u]) continue ;
        if(dfn[v]) {
            if(dfn[v] < dfn[u]) continue ;
            loop[++ cnt] = v;
            for ( ; v != u; v = fa[v])
                loop[++ cnt] = fa[v];
        } else fa[v] = u, get_loop(v);
    }
}