洛谷P3385负环

传送门

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <queue>
#define re register 
using namespace std;
const int maxn = 2005;
const int maxm = 3005;

inline int read(){
	char ch = getchar();
	int f = 1 , x = 0 ;
	while(ch > '9' || ch < '0') {if(ch == '-' ) f = -1 ; ch = getchar();}
	while(ch >= '0' && ch <= '9'){x = (x << 1) + (x << 3) + ch - '0' ;ch = getchar();}
	return x * f;
}

int T,n,m,u,v,w;
int head[maxn],tot;
bool flag;
 
struct Edge{
	int from , to , next , val;
}edge[maxm << 1];

inline void add(int u , int v , int w){
	edge[++tot].from = u ;
	edge[tot].to = v;
	edge[tot].val = w;
	edge[tot].next = head[u];
	head[u] = tot;
}

int dis[maxn],num[maxn];
bool vis[maxn];

inline bool spfa(int s){
	for(re int i = 1 ; i <= n ; ++i)  dis[i] = 1e9 ;
	queue<int> q;
	q.push(s);
	dis[s] = 0;
	num[s]++;
	vis[s] = true;
	while(!q.empty()){
		int cur = q.front();
		q.pop(); vis[cur] = 0;
		for(re int i = head[cur] ; i ; i = edge[i].next) {
			int v = edge[i].to ;
			if(dis[v] > dis[cur] + edge[i].val) {
				dis[v] = dis[cur] + edge[i].val;
				if(!vis[v]) {
					q.push(v);
					num[v]++;
					vis[v] = true;
					if(num[v] > n)  return false;
				}
			}
		}
	}
	return true;
}			

int main(){
	T = read();
	while(T--){
		memset(dis , 0 , sizeof(dis));
		memset(edge , 0 ,sizeof(edge));
		memset(num , 0 , sizeof(num));
		memset(head , 0 , sizeof(head));
		memset(vis , 0 , sizeof(vis));
		tot = 0;
		flag = 0;
		n = read(); m = read();
		for(re int i = 1 ; i <= m ; ++i){
			u = read(); v = read(); w = read();
			if(w < 0) add(u , v , w);
			else {
				add(u , v , w);
				add(v , u , w);
			}
		}
		if(spfa(1)) printf("N0
");
		else printf("YE5
");
	}
	return 0;
}