点双连通分支（按边分块）

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int tol,head[maxn];
struct edge
{
    int to,id,next;
}es[maxm];
void addedge( int u , int v , int d )
{
    es[tol].to = v;
    es[tol].id = d;
    es[tol].next = head[u];
    head[u] = tol++;
}
int Low[maxn],Dfn[maxn],Stack[maxm],Belong[maxm];
int Index,Top,Block;
void dfs( int u , int f )
{
    int v,line;
    Low[u] = Dfn[u] = ++Index;
    for ( int i=head[u] ; i!=-1 ; i=es[i].next )
    {
        v = es[i].to,line = es[i].id;
        if ( v==f||Belong[line] ) continue;
        if ( !Dfn[v] )
        {
            Stack[Top++] = line;
            dfs( v , u );
            if ( Low[u]>Low[v] ) Low[u] = Low[v];
            if ( Low[v]>=Dfn[u] )
            {
                Block++;
                int linen;
                do
                {
                    linen = Stack[--Top];
                    Belong[linen] = Block;
                }while( linen!=line );
            }
        }
        else if ( Dfn[v]<Dfn[u] )
        {
            Stack[Top++] = line;
            if ( Dfn[v]<Low[u] ) Low[u] = Dfn[v];
        }
    }
}
void Tarjan()
{
    memset ( Dfn , 0 , sizeof(Dfn) );
    memset ( Belong , 0 , sizeof(Belong) );
    Index = Block = Top = 0;
    for ( int i=1 ; i<=n ; i++ )
        if ( !Dfn[i] ) dfs( i , i );
}