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差分约束问题（判定负环）

 

 

#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
#include<queue>
using namespace std;
const int INF = 1e9;
const int maxn = 300;

struct Node {
	int p;
	int next;
	int len;
}G[5000];
int z = 0;
int head[maxn];
void add(int be, int en, int len) {
	G[++z].p = en;
	G[z].len = len;
	G[z].next = head[be];
	head[be] = z;
}
int n, m;
int vis[maxn];
int cnt[maxn];
int dis[maxn];
int gi() {
	int a = 0; char x = getchar(); bool f = 0;
	while ((x<'0' || x>'9') && x != '-')x = getchar();
	if (x == '-')x = getchar(), f = 1;
	while (x >= '0'&&x <= '9')a = (a << 3) + (a << 1) + x - 48, x = getchar();
	return f ? -a : a;
}

int spfa(int be) {
	queue<int>que;
	for (int i = 0; i <= n; i++) dis[i] = INF;
	dis[be] = 0;
	vis[be] = 1;
	que.push(be);

	while (que.size()) {
		int  x = que.front();
		que.pop();
		vis[x] = 0;
		++cnt[x];
		if (cnt[x] == n) return 0;//有错
		
		for (int i = head[x]; i; i = G[i].next) {
			int p = G[i].p;

			if (dis[p] > dis[x] + G[i].len) {
				dis[p] = dis[x] + G[i].len;
				if (!vis[p] ) {
					que.push(p);
					vis[p] = 1;
				}
			}
		}
	}
	return 1;
}
int main() {
	int t;
	t = gi();
	while (t--) {
		n = gi();
		m =  gi();
		int be, en, len;
		memset(head, 0, sizeof(head));
		z = 0;
		memset(vis, 0, sizeof(vis));
		memset(cnt, 0, sizeof(cnt));

		for (int i = 0; i < m; i++) {
			be = gi();
			en = gi();
			len = gi();
			add(be - 1, en, len);
			add(en, be - 1, -len);
		}
		int flag = 0;
		for (int i = 0; i <= n; i++) {
			if (!cnt[i] ) {
				if (!spfa(i)) {
					flag = 1;
					break;
				}
			}
		}
		if (!flag) printf("true\n");
		else printf("false\n");
	}
	return 0;
}