SPFA判負環

馬上就退役了，時間不足就不多介紹了

反正DFS是會T飛的，BFS就沒關係了qwq

#include<cmath>
#include<queue>
#include<cstdio>
#include<iostream>
#include<algorithm>
#define rg register
#ifdef _WIN32
#define lld "I64d"
#else 
#define lld "%lld"
#endif
using namespace std;
inline long long read(){
	long long a=0; int f=0; char c=getchar();
	while(c<'0'||c>'9') { f|=c=='-'; c=getchar(); }
	while(c>='0'&&c<='9') { a=(a<<3)+(a<<1)+(c^48);	c=getchar(); }
	return f? -a:a;
}
int t,n,m,fir[2002],dis[2002],cnt,q[6000006],head,tail,num[2002];
bool vis[2002],flag;
struct edge{int v,w,nxt;} e[6006];
void add(int x,int y,int z){
	e[++cnt].v=y,e[cnt].w=z,e[cnt].nxt=fir[x],fir[x]=cnt;
}
bool spfa(){
	while(head<tail){
		int u=q[++head]; vis[u]=0;
		if(num[u]>n) return 1;
		for(int i=fir[u],v;v=e[i].v,i;i=e[i].nxt){
			if(dis[v]>dis[u]+e[i].w){
				dis[v]=dis[u]+e[i].w,num[v]=num[u]+1;
				if(num[v]>n) return 1;
				if(!vis[v]) vis[v]=1,q[++tail]=v;
			}
		}
	}
	return 0;
}
int main(){
//	freopen("testdata.in","r",stdin);
//	freopen("qaq.txt","w",stdout);
	t=read();
	while(t--){
		n=read(),m=read();
		for(int i=1;i<=n;++i) dis[i]=214748364,q[i]=0,vis[i]=0,fir[i]=0;
		cnt=0,q[1]=1,head=0,tail=1,num[1]=vis[1]=1,dis[1]=0;
		for(int i=1,x,y,z;i<=m;++i){
			x=read(),y=read(),z=read(),add(x,y,z);
			if(z>=0) add(y,x,z);
		}
		if(spfa()) printf("YE5\n");
		else printf("N0\n");
	}
	return 0;
}