[HNOI2005]狡猾的商人 差分约束+判环

#include <bits/stdc++.h>
using namespace std;
#define INF 0x7fffffff
struct Edge{
	int v,w,nxt;
};
int cnt,head[100000];
Edge edge[100000];
int book[10005],dis[10005],tot[100005],vis[100005],n,m;
int read(){
	int x=0,dign=1;
	char c = getchar();
	while(c<'0'||c>'9'){
		if(c=='-')dign=-1;
		c = getchar();
	}
	while(c>='0'&&c<='9'){
		x = x*10 + c - '0';
		c = getchar();
	}
	return x*dign;
}
void add(int u,int v,int w){
	cnt++;
	edge[cnt].v = v; edge[cnt].w = w;
	edge[cnt].nxt = head[u];
	head[u] = cnt;
}
bool spfa(int u){
	queue<int> q;
	for(int i=1;i<=n;i++)dis[i] = INF;
	dis[u] = 0; book[u] = 1; q.push(u);
	while(!q.empty()){
		int u = q.front(); book[u] = 0; q.pop();
		tot[u]++; if(tot[u]==n)return false; 
		for(int v,w,i = head[u];i;i=edge[i].nxt){
			v = edge[i].v; w = dis[u] + edge[i].w;
			if(dis[v]>w){
				dis[v] = w;
				if(!book[v]){
					book[v]=1; q.push(v);
				}
			} 
		}
	}
	return true;
}
int main()
{
	int w;
	w = read();
	while(w--){
	cnt = 0; memset(book,0,sizeof(book));
	memset(tot,0,sizeof(tot));
	memset(head,0,sizeof(head));
	n = read(); m = read();
	
	for(int i=0;i<m;i++){
		int a,b,c;
		a = read(); b = read(); c = read();
		add(a-1,b,c);
		add(b,a-1,-c);
	}
	int f = 1;
	for(int i=0;i<=n;i++){
		if(!tot[i]){
			f = spfa(i);if(!f)break;
		}
	}
	if(!f)printf("false\n");
	else printf("true\n"); 
	}
	return 0;
 }