UVa 11090 Going in Cycle!!（环上平均值最大+SPFA DFS判负环）

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题意：给定一个n个点m条边的加权有向图，求平均权值最小的回路。

分析：二分+负权回路的判断，数据居然有自环，MDZZ,WA了一晚上。

#include <cstdio>#include <stack>#include <queue>    #include <vector>    #include <cstdio>    #include <utility>    #include <cstring>    #include <iostream>    #include <algorithm>    #define INF 0x3f3f3f3f#define eps 1e-6using namespace std;int T,n,m,num;long long tot;typedef pair <int,double> Pii;bool jud[105]; double dis[105];vector <Pii> G[105];bool dfs(int u,double k){jud[u] = true;for(Pii x : G[u]){int v = x.first;double val = x.second;if(dis[u] + val - k < dis[v]) {dis[v] = dis[u] + val - k;if(jud[v]) return true;if(dfs(v,k)) return true;}}jud[u] = false;return false;}bool Jud(double k){for(int i = 1;i <= n;i++) dis[i] = 0;memset(jud,0,sizeof(jud));for(int i = 1;i <= n;i++) if(dfs(i,k)) return true;return false;}int main(){scanf("%d",&T);while(T--){tot = 0;scanf("%d%d",&n,&m);for(int i = 1;i <= n;i++) G[i].clear();while(m--){int u,v,val;scanf("%d%d%d",&u,&v,&val);G[u].push_back(make_pair(v,val));tot += val;}double l = 0,r = tot + 1;while(r - l > eps){double mid = (l+r)/2.0;if(Jud(mid)) r = mid;else l = mid;} printf("Case #%d: ",++num);if(!Jud(r)) printf("No cycle found.\n");else printf("%.2lf\n",l);}}

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