UVa 11090 Going in Cycle!!【Bellman_Ford】

  1 #include<iostream>  
  2 #include<cstdio>  
  3 #include<cstring> 
  4 #include <cmath> 
  5 #include<stack>
  6 #include<vector>
  7 #include<map> 
  8 #include<set>
  9 #include<queue> 
 10 #include<algorithm>  
 11 #define mod=1e9+7;

 12 using namespace std;
 13 
 14 typedef long long LL;
 15 const int INF = 0x7fffffff;
 16 const int maxn=10005;
 17 
 18 struct Edge{
 19     int from,to;

 20     double dist;
 21 };
 22 
 23 struct BellmanFord{
 24     int n,m;
 25     vector<Edge> edges;
 26     vector<int> G[maxn];
 27     bool inq[maxn];
 28     double d[maxn];
 29     int p[maxn];
 30     int cnt[maxn];
 31     
 32     void init(int n){
 33         this->n=n;
 34         for(int i=0;i<n;i++) G[i].clear();
 35         edges.clear();
 36     }
 37     
 38     void AddEdges(int from,int to,double dist){
 39         edges.push_back((Edge){from,to,dist});
 40         m=edges.size();
 41         G[from].push_back(m-1);
 42     }
 43     
 44     bool negativeCycle(){
 45         queue<int> Q;
 46         memset(inq,0,sizeof(inq));
 47         memset(cnt,0,sizeof(cnt));
 48         for(int i=0;i<n;i++) {d[i]=0;inq[0]=true;Q.push(i);}
 49         
 50         while(!Q.empty()){
 51             int u=Q.front();Q.pop();
 52             inq[u]=false;
 53             for(int i=0;i<G[u].size();i++){
 54                 Edge& e=edges[G[u][i]];
 55                 if(d[e.to]>d[u]+e.dist){
 56                     d[e.to]=d[u]+e.dist;
 57                     p[e.to]=G[u][i];
 58                     if(!inq[e.to]){
 59                         Q.push(e.to);
 60                         inq[e.to]=true;
 61                         if(++cnt[e.to]>n) 
 62                         return true;
 63                     }
 64                 }
 65             }
 66         }
 67         return false;
 68     }
 69 };
 70 
 71 BellmanFord solver;
 72 
 73 bool test(double x){
 74     for(int i=0;i<solver.m;i++)
 75     solver.edges[i].dist-=x;
 76     
 77     bool ret=solver.negativeCycle();
 78     for(int i=0;i<solver.m;i++)
 79     solver.edges[i].dist+=x;
 80     return ret;
 81 }
 82 
 83 int main(){
 84     int T;
 85     scanf("%d",&T);
 86     for(int kase=1;kase<=T;kase++){
 87         int n,m;
 88         scanf("%d %d",&n,&m);
 89         solver.init(n);
 90         int ub=0;
 91         while(m--){
 92             int u,v,w;
 93             scanf("%d %d %d",&u,&v,&w);u--;v--;ub=max(ub,w);
 94             solver.AddEdges(u,v,w);            
 95         }
 96         printf("Case #%d: ",kase);
 97         if(!test(ub+1)) printf("No cycle found.\n");
 98         else{
 99             double L=0,R=ub;
100             while(R-L>1e-3){
101                 double M=L+(R-L)/2;
102                 if(test(M)) R=M;else L=M;
103             }
104             printf("%.2lf\n",L);
105         }
106     }
107     return 0;
108 }