PKU 3259

SPFA找负环，当一个点入队的次数>=n时就证明出现了负环。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <queue>
using namespace std;
const int  N =510;
vector <int> map[N];
struct Edge
{
    int to,dis;
    Edge * next;
};
Edge *adj[N];
Edge edges[N*N*3];
int cnt;
void addEdge(int s,int e,int cost)
{
    Edge *ptr = &edges[cnt++];
    ptr->to =e;
    ptr->dis = cost;
    ptr->next = adj[s];
    adj[s] = ptr;
}
int n,m;
bool vis[N];
int vCnt[N];
int dis[N];
bool spfa()
{
    queue <int> Q;
    memset(vis,false,sizeof(vis));
    memset(vCnt,0,sizeof(vCnt));
    memset(dis,127,sizeof(dis));

    Q.push(1);
    vCnt[1]++;
    dis[1] = 0;
    while(!Q.empty())
    {
        int u = Q.front();Q.pop();
        vis[u] = false;
        for(Edge *ptr =adj[u];ptr;ptr=ptr->next)
        {
            int e = ptr->to;
            int cost = ptr->dis;
            if(dis[e]>dis[u]+cost)
            {
                dis[e] = dis[u] + cost;
                if(!vis[e])
                {
                    Q.push(e);
                    vis[e] = true;
                    vCnt[e]++;
                    if(vCnt[e]>=n)
                    {
                        return true;
                    }
                }
            }
        }
    }
    return false;

}
int main()
{
    int F,w;
    scanf("%d",&F);
    while(F--)
    {
        memset(adj,0,sizeof(adj));
        cnt = 0;
        scanf("%d%d%d",&n,&m,&w);
        int s,e,cost;
        for(int i=1;i<=m;i++)
        {
            scanf("%d%d%d",&s,&e,&cost);
            addEdge(s,e,cost);
            addEdge(e,s,cost);
        }
        for(int i=1;i<=w;i++)
        {
            scanf("%d%d%d",&s,&e,&cost);
            addEdge(s,e,-cost);
        }
        spfa()?printf("YES\n"):printf("NO\n");
    }
    return 0;
}