題意：有n個點。一個點到另外一個點上需要時間，由有一些蟲洞，你穿過蟲洞，可以返回多少秒之前，問是否能無限返回。

題解：判斷負環=-=，spfa直接解決。

Select Code
#include<cstring>
#include<cstdio>
#include<queue>
#include<cmath>
#include<algorithm>
#include<iostream>
using namespace std;

const int maxn =550;
const int maxm = 5550;
const int INF = 1e9;
struct Edge{
	int to;
	int w;
	int next;
}edge[maxm];
int head[maxn];
int mark[maxn];
int vis[maxn];
int dis[maxn];
int tol;
int n,m,k;
void add(int u,int v,int w){
	edge[tol].to = v;
	edge[tol].w = w;
	edge[tol].next = head[u];
	head[u] = tol++;
}
bool spfa(){
	for(int i = 1 ; i <= n ; i++){
		mark[i] = 0;
		dis[i] = INF;
		vis[i] = 0;
	}
	queue<int> q;
	q.push(1);
	dis[1] = 0;
	vis[1] = 1;
	mark[1] = 1;
	while(!q.empty()){
		int u = q.front();
		q.pop();
		mark[u] = 0;
		for(int i = head[u] ; i != -1 ; i = edge[i].next){
			int v = edge[i].to;
			if(dis[v] > dis[u] + edge[i].w){
				dis[v] = dis[u] + edge[i].w;
				if(!mark[v]){
					mark[v] =1;
					vis[v]++;
					q.push(v);
					if(vis[v] >= n){
						return true;
					}
				}
			}
		}
	}
	return false;
}
int main(){
	int z;
	cin >> z;
	while(z--){
		memset(head,-1,sizeof(head));
		
		tol = 0;
		cin >> n >> m >> k;
		while(m--){
			int u,v,w;
			cin >> u >> v >> w;
			add(u,v,w);
			add(v,u,w);
		}
		while(k--){
			int u,v,w;
			cin >> u >> v >> w;
			add(u,v,-w);
		}
		if(spfa())
			cout << "YES" << endl;
		else 
			cout << "NO" << endl;
	}
}