USACO butter

　　多次使用dijkstra就行， 代码如下：

　　

/*
    ID: m1500293
    LANG: C++
    PROG: butter
*/

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <queue>
#include <vector>

using namespace std;
int N, P, C;           //牛的数量 牧场的数量 边的数量
int cows[550];

struct edge
{
    int to, cost;
    edge() {}
    edge(int to, int cost):to(to), cost(cost) {}
};
vector<edge> G[810];

struct Dij
{
    int u, cost;
    Dij() {}
    Dij(int u, int cost):u(u), cost(cost) {}
    bool operator<(const Dij &r) const
    {
        return cost > r.cost;
    }
};

int dist[810], vis[810];
void dijkstra(int s)
{
    memset(vis, 0, sizeof(vis));
    memset(dist, 0x3f, sizeof(dist));
    priority_queue<Dij> que;
    dist[s] = 0;
    que.push(Dij(s, 0));
    while(!que.empty())
    {
        Dij u = que.top(); que.pop();
        if(vis[u.u]) continue;
        vis[u.u] = 1;
        for(int i=0; i<G[u.u].size(); i++)
        {
            edge e = G[u.u][i];
            if(dist[e.to] > dist[u.u]+e.cost)
            {
                dist[e.to] = dist[u.u]+e.cost;
                que.push(Dij(e.to, dist[e.to]));
            }
        }
    }
}

int main()
{
    freopen("butter.in", "r", stdin);
    freopen("butter.out", "w", stdout);
    scanf("%d%d%d", &N, &P, &C);
    for(int i=0; i<N; i++)
        scanf("%d", &cows[i]);
    for(int i=0; i<C; i++)
    {
        int u, v, cost;
        scanf("%d%d%d", &u, &v, &cost);
        G[u].push_back(edge(v, cost));
        G[v].push_back(edge(u, cost));
    }
    int sum=0x3fffffff;
    for(int i=1; i<=P; i++)
    {
        dijkstra(i);
        int tp = 0;
        for(int j=0; j<N; j++)
        {
            tp += dist[cows[j]];
//            if(dist[cows[j]] == 0x3f3f3f3f)
//            {
//                tp = -1;
//                break;
//            }
        }
//        if(tp < 0) continue;
        if(tp < sum)
            sum = tp;
    }
    printf("%d\n", sum);
    return 0;
}