最短路问题，用Bellman算法判断有无正向环。
1 #include <stdio.h> 2 double M[105],orig; 3 int m,n,s; 4 typedef struct 5 { 6 int u,v; 7 double r,c; 8 void init(int a,int b,double q,double p) 9 {u = a; v = b; r = q; c = p;} 10 }Edge; 11 Edge ed[205]; 12 void read_graph() 13 { 14 int a,b,i; 15 double q,w,e,r; 16 for(i = 1; i <= n; i++) 17 M[i] = 0.0; 18 for(i = 0; i < m; i++) 19 { 20 scanf("%d%d%lf%lf%lf%lf",&a,&b,&q,&w,&e,&r); 21 ed[i<<1].init(a,b,q,w); 22 ed[i<<1|1].init(b,a,e,r); 23 } 24 } 25 bool Bellman() 26 { 27 int i,j,u,v; 28 double r,c; 29 bool ok; 30 M[s] = orig; 31 for(j = 1; j < n; j++) 32 { 33 ok = 1; 34 for(i = 0; i < 2*m; i++) 35 { 36 u = ed[i].u; v = ed[i].v; 37 r = ed[i].r; c = ed[i].c; 38 if((M[u]-c)*r > M[v]) 39 {M[v] = (M[u]-c)*r; ok = 0;} 40 } 41 if(ok) break; 42 } 43 for(i = 0; i < 2*m; i++) 44 { 45 u = ed[i].u; v = ed[i].v; 46 r = ed[i].r; c = ed[i].c; 47 if((M[u]-c)*r > M[v]) 48 return 1; 49 } 50 return 0; 51 } 52 int main() 53 { 54 while(~scanf("%d%d%d%lf",&n,&m,&s,&orig)) 55 { 56 read_graph(); 57 if(Bellman()) 58 printf("YES\n"); 59 else printf("NO\n"); 60 } 61 return 0; 62 }