1 //bellman_ford判断负环 2 #include <vector> 3 #include <iostream> 4 #include <cstring> 5 #include <algorithm> 6 using namespace std; 7 8 const int maxn = 200 + 5; 9 10 int n,m; 11 double v; 12 struct node{ 13 int u, v; 14 double r,c; 15 }p[maxn]; 16 17 double dis[maxn]; 18 int cnt; 19 20 bool bellman_ford(int x){ 21 memset(dis, 0, sizeof(dis)); 22 dis[x] = v; 23 for(int i = 1;i < n;i++){ 24 bool flag = true; 25 for(int j = 0;j < cnt;j++){ 26 int aa = p[j].u; 27 int bb = p[j].v; 28 double rr = p[j].r; 29 double cc = p[j].c; 30 if(dis[bb] < (dis[aa] - cc)*rr){ 31 dis[bb] = (dis[aa] - cc)*rr; 32 flag = false; 33 } 34 } 35 if(flag) 36 break; 37 } 38 39 //判断出现负环 40 for(int i = 0; i< cnt;i++) 41 if( dis[p[i].v] < (dis[p[i].u]-p[i].c)*p[i].r) 42 return true; 43 return false; 44 } 45 46 int main(){ 47 int s; 48 cin >> n >> m >> s >>v; 49 cnt = 0; 50 while(m--){ 51 int a, b; 52 double c1,r1,c2,r2; 53 cin >> a >> b >> c1 >> r1 >> c2 >> r2; 54 p[cnt].u = a, p[cnt].v = b, p[cnt].r = c1, p[cnt++].c = r1; 55 p[cnt].u = b, p[cnt].v = a, p[cnt].r = c2, p[cnt++].c = r2; 56 } 57 if(bellman_ford(s)) 58 cout << "YES" << endl; 59 else 60 cout << "NO" << endl; 61 62 return 0; 63 }