要求路径长度大于一，判断出环即可结束。。
1 #include<iostream> 2 #include<cstdio> 3 #include<cstdlib> 4 #include<cstring> 5 #include<string> 6 #include<queue> 7 #include<algorithm> 8 #include<map> 9 #include<iomanip> 10 #include<climits> 11 #include<string.h> 12 #include<cmath> 13 #include<stdlib.h> 14 #include<vector> 15 #include<set> 16 #define INF 1e7 17 #define MAXN 100010 18 #define maxn 1000010 19 #define Mod 1000007 20 #define N 1010 21 using namespace std; 22 typedef long long LL; 23 24 struct node{ int x; double w; }; 25 vector<node> G[1010]; 26 int u, v; 27 double w; 28 int n, m; 29 int T; 30 int cnt[555]; 31 int vis[555]; 32 double dis[555]; 33 34 bool spfa() 35 { 36 queue<node> q; 37 q.push({ 1, 0 }); 38 vis[1] = 1; 39 dis[1] = 1; 40 while (!q.empty()){ 41 node now = q.front(); 42 q.pop(); 43 vis[now.x] = 0; 44 for (int i = 0; i < G[now.x].size(); ++i) { 45 node v = G[now.x][i]; 46 if (dis[now.x] * v.w > dis[v.x]) { 47 dis[v.x] = dis[now.x] * v.w; 48 if (!vis[v.x]) { 49 vis[v.x] = 1; 50 cnt[v.x]++; 51 q.push({ v.x, dis[v.x] }); 52 if (cnt[v.x] >= n) return true; 53 } 54 } 55 } 56 } 57 return false; 58 } 59 60 void init() 61 { 62 memset(dis,0,sizeof(dis)); 63 memset(vis,0,sizeof(vis)); 64 memset(cnt,0,sizeof(cnt)); 65 for (int i = 0; i <= n; ++i) 66 G[i].clear(); 67 } 68 69 void run() 70 { 71 cin >> n >> m; 72 init(); 73 for (int i = 0; i < m; ++i) { 74 cin >> u >> w >> v; 75 G[u].push_back({v,w}); 76 } 77 if (spfa()) puts("Yes"); 78 else puts("No"); 79 } 80 81 int main() 82 { 83 cin >> T; 84 while (T--) 85 run(); 86 //system("pause"); 87 return 0; 88 }