POJ3259(spfa判负环)

思路：如果某个顶点入队列的次数超过了n，那么久存在负环！ 记得清空队列

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <queue>
#include <algorithm>
using namespace std;
#define inf 9999999
#define N 520
#define M 5520
int size,n,m,l;
int dis[N],vis[N],head[N],in[N];
struct Edge
{
    int v,w,next;
    Edge(){}
    Edge(int V,int W,int NEXT):v(V),w(W),next(NEXT){}
}edge[M];

void Init()
{
    size = 0;
    memset(head,-1,sizeof(head));
}

void InsertEdge(int u,int v,int w)
{
    edge[size] = Edge(v,w,head[u]);
    head[u] = size++;
}

bool spfa(int st)
{
    for(int i=1; i<=n; i++)
    {
        vis[i] = 0;
        dis[i] = inf;
        in[i] = 0;
    }
    dis[st] = 0 ; vis[st] = 1;
    queue<int> Q;
    while(!Q.empty()) Q.pop();
    Q.push(st);
    in[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;
                    in[v]++;
                    if(in[v] > n) return 1;
                    Q.push(v);
                }
            }
        }
    }
    return 0;
}

int main()
{
    int T;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d%d%d",&n,&m,&l);
        Init();
        int u,v,w;
        int flag ;
        for(int i=0; i<m; i++)
        {
            scanf("%d%d%d",&u,&v,&w);
            InsertEdge(u,v,w);
            InsertEdge(v,u,w);
        }
        for(int i=0; i<l; i++)
        {
            scanf("%d%d%d",&u,&v,&w);
            InsertEdge(u,v,-w);
        }
        if(spfa(1)) printf("YES
");
        else printf("NO
");
    }
    return 0;
}