48.spfa_negative_ring.cpp

#include<iostream>
#include<cstring>
#include<queue>
using namespace std;
const int N = 2e3 + 10, M = 1e4 + 10;
int n, m;
int h[N], e[M], ne[M], w[M], idx;
bool st[N];
int dist[N];
int cnt[N];

void add(int a, int b, int c){
	e[idx] = b, w[idx] = c, ne[idx] = h[a], h[a] = idx++;
}
bool spfa(){
	queue<int> q;
	for(int i = 1; i <= n; i++){
		q.push(i);
		st[i] = true;
	}
	while(q.size()){
		int t = q.front();
		q.pop();
		st[t] = false;
		for(int i = h[t]; i != -1; i = ne[i]){
			int j = e[i];
			if(dist[j] > dist[t] + w[i]){
				dist[j] = dist[t] + w[i];
				cnt[j] = cnt[t] + 1;
				if(cnt[j] >= n)	return true;
				if(!st[j]){
					q.push(j);
					st[j] = true;
				}
			}
		}
	}
	return false;
}
int main()
{
	cin >> n >> m;
	memset(h, -1, sizeof h);
	for(int i = 0; i < m; i++){
		int a, b, c;
		cin >> a >> b >> c;
		add(a, b, c);
	}
	if(spfa()){
		cout << "Yes" << endl;
	}
	else	cout << "No" << endl;
	return 0;
}