第一问是求最小生成树，第二问是求次小生成树。如果次小生成树不存在（本身就是一棵树），则输出-1。

/* 
 * Problem: Ural1416 Confidential
 * Author: Guo Jiabao
 * Time: 2009.4.14 9:34
 * State: Solved
 * Memo: 次小生成树 Prim
*/
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>
const int MAXN=501,MAXM=MAXN*MAXN*4,INF=0x7FFFFFFF;
using namespace std;
struct edge
{
    edge *next,*op;
    int t,c;
    bool mst;
}ES[MAXM],*V[MAXN],*MST[MAXN],*CLS[MAXN];
int N,M,EC=-1,MinST,Ans,NMST;
int DM[MAXN],F[MAXN];
inline void addedge(edge **V,int a,int b,int c)
{
    ES[++EC].next=V[a];
    V[a]=ES+EC; V[a]->t=b; V[a]->c=c;
    ES[++EC].next=V[b];
    V[b]=ES+EC; V[b]->t=a; V[b]->c=c;
    V[a]->op=V[b]; V[b]->op=V[a];
    V[a]->mst=V[b]->mst=false;
}
void init()
{
    int i,a,b,c;
    freopen("conf.in","r",stdin);
    freopen("conf.out","w",stdout);
    scanf("%d%d",&N,&M);
    for (i=1;i<=M;i++)
    {
        scanf("%d%d%d",&a,&b,&c);
        addedge(V,a,b,c);
    }
    Ans=INF;
}
void prim()
{
    int i,j,Mini;
    for (i=1;i<=N;i++)
        DM[i]=INF;
    for (i=1;i;)
    {
        DM[i]=-INF;
        for (edge *e=V[i];e;e=e->next)
        {
            if (e->c < DM[j=e->t])
            {
                DM[j]=e->c;
                CLS[j]=e->op;
            }
        }
        Mini=INF;i=0;
        for (j=1;j<=N;j++)
            if (DM[j]!=-INF && DM[j]<Mini)
            {
                Mini=DM[j];
                i=j;
            }
    }
    for (i=2;i<=N;i++)
    {
        MinST+=CLS[i]->c;
        CLS[i]->mst=CLS[i]->op->mst=true;
        addedge(MST,CLS[i]->t,CLS[i]->op->t,CLS[i]->c);
    }
}
void dfs(int i)
{
    for (edge *e=MST[i];e;e=e->next)
    {
        int j=e->t;
        if (F[j]==-INF)
        {
            F[j]=F[i];
            if (e->c > F[j])
                F[j]=e->c;
            dfs(j);
        }
    }
}
void smst()
{
    int i,j;
    for (i=1;i<=N;i++)
    {
        for (j=1;j<=N;j++)
            F[j]=-INF;
        F[i]++;
        dfs(i);
        for (edge *e=V[i];e;e=e->next)
        {
            if (!e->mst)
            {
                NMST=MinST + e->c -F[e->t];
                if (NMST < Ans)
                    Ans=NMST;
            }
        }
    }
}
int main()
{
    init();
    prim();
    smst();
    if (Ans==INF)
        Ans=-1;
    printf("Cost: %dnCost: %dn",MinST,Ans);
    return 0;
}