if(!dfn[v])
{
    son++;
    tarjan(v, x);
    low[x] = min(low[x], low[v]);
    if(x != fa && low[v] >= dfn[x] && is[x] == 0) //是is[x]而不是is[v]!!!
    {
        is[x] = 1;
        p++;
    }
}