POJ3268

题意：给你n个奶牛，m个农场，在x农场开派对，问你所有奶牛中必须花费最大的时间

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <queue>
using namespace std;
#define M 100005
#define N 1005
#define inf 9999999
int size1,size2,n,m,x;
int dis[N],vis[N],head1[N],head2[N];
int sum[N];
struct Edge
{
    int v,w,next;
    Edge(){}
    Edge(int V,int W,int NEXT):v(V),w(W),next(NEXT){}
}edge1[M],edge2[M];

void Init()
{
    size1 = 0;
    size2 = 0;
    memset(head1,-1,sizeof(head1));
    memset(head2,-1,sizeof(head2));
}

void InsertEdge1(int u,int v,int w)
{
    edge1[size1] = Edge(v,w,head1[u]);
    head1[u] = size1++;
}
void InsertEdge2(int u,int v,int w)
{
    edge2[size2] = Edge(v,w,head2[u]);
    head2[u] = size2++;
}

void spfa(int st,Edge edge[],int head[])
{
    for(int i=1; i<=n; i++)
    {
        vis[i] = 0;
        dis[i] = inf;
    }
    dis[st] = 0; vis[st] = 1;
    queue<int> Q;
    Q.push(st);
    while(!Q.empty())
    {
        int u = Q.front();
        Q.pop();
        vis[u] = 0;
        for(int i=head[u]; i!=-1; i = edge[i].next)
        {
            int v = edge[i].v;
            if(dis[v] > dis[u] + edge[i].w)
            {
                dis[v] = dis[u] + edge[i].w ;
                if(!vis[v])
                {
                    vis[v] = 1;
                    Q.push(v);
                }
            }
        }
    }
}

int main()
{
    while(scanf("%d%d%d",&n,&m,&x)!=EOF)
    {
        Init();
        int u,v,w;
        for(int i=0; i<m; i++)
        {
            scanf("%d%d%d",&u,&v,&w);
            InsertEdge1(u,v,w);
            InsertEdge2(v,u,w);
        }
        memset(sum,0,sizeof(sum));
        spfa(x,edge1,head1);
        for(int i=1; i<=n; i++)
        sum[i] += dis[i];
        spfa(x,edge2,head2);
        for(int i=1; i<=n; i++)
        sum[i] += dis[i];
        int ans = -1;
        for(int i=1; i<=n; i++)
        ans = max(ans,sum[i]);
        printf("%d
",ans);
    } 
    return 0;
}