bzoj 1415 期望+记忆化搜索 ****

  1 #include<cstdio>
  2 #include<iostream>
  3 #include<algorithm>
  4 #include<cstring>
  5 #include<cmath>
  6 #include<queue>
  7 #include<map>
  8 using namespace std;
  9 #define MOD 1000000007
 10 const int INF=0x3f3f3f3f;
 11 const double eps=1e-5;
 12 typedef long long ll;
 13 #define cl(a) memset(a,0,sizeof(a))
 14 #define ts printf("*****\n");
 15 int n,m,tt;
 16 const int MAXN = 1010;
 17 const int MAXM = 1010*4;
 18 int mp[MAXN][MAXN];
 19 struct Edge
 20 {
 21     int to,next;
 22 }edge[MAXM];
 23 int head[MAXN],tot;
 24 void addedge(int u,int v)
 25 {
 26     edge[tot].to = v;
 27     edge[tot].next = head[u];
 28     head[u] = tot++;
 29 
 30 }
 31 
 32 void init()
 33 {
 34     tot = 0;
 35     memset(head,-1,sizeof(head));
 36 }
 37 
 38 void bfs(int st)
 39 {
 40     queue<int> q;
 41     q.push(st);
 42     mp[st][st]=0;
 43     while(!q.empty())
 44     {
 45         int u=q.front();
 46         q.pop();
 47         for(int i=head[u];i!=-1;i=edge[i].next)
 48         {
 49             int v=edge[i].to;
 50             if(v==u)    continue;
 51             if(mp[st][v]==-1)
 52             {
 53                 mp[st][v]=mp[st][u]+1;
 54                 q.push(v);
 55             }
 56         }
 57     }
 58 }
 59 int to[MAXN][MAXN];
 60 double f[MAXN][MAXN];
 61 bool vis[MAXN][MAXN];
 62 double F(int x,int y)
 63 {
 64     if(vis[x][y])   return f[x][y];
 65     vis[x][y]=1;
 66     if(mp[x][y]==0)    return f[x][y]=0.0;
 67     if(mp[x][y]<=2)    return f[x][y]=1.0;
 68     double val=0;
 69     int cnt=0;
 70     int v;
 71     for(int i=head[y];i!=-1;i=edge[i].next)
 72     {
 73         cnt++;
 74         v=edge[i].to;
 75         val+=F(to[to[x][y]][y],v);
 76     }
 77     return f[x][y]=val/cnt+1.0;
 78 }
 79 void fun()
 80 {
 81     for(int i=1;i<=n;i++)
 82     {
 83         for(int j=1;j<=n;j++)
 84         {
 85             printf("%.2lf ",f[i][j]);
 86         }
 87         printf("\n");
 88     }
 89 }
 90 int main()
 91 {
 92     int s,t;
 93     int i,j,k;
 94     #ifndef ONLINE_JUDGE
 95     freopen("1.in","r",stdin);
 96     #endif
 97     init();
 98     scanf("%d%d%d%d",&n,&m,&s,&t);
 99     int u,v;
100     for(i=0;i<m;i++)
101     {
102         scanf("%d%d",&u,&v);
103         addedge(u,v);
104         addedge(v,u);
105     }
106     memset(mp,-1,sizeof(mp));
107     int mx;
108     for(k=1;k<=n;k++)
109     {
110         bfs(k);
111         for(j=1;j<=n;j++)
112         {
113             if(j==k)    continue;
114             mx=INF;
115             for(i=head[j];i!=-1;i=edge[i].next)
116             {
117                 int v=edge[i].to;
118                 if(mx>mp[k][v])
119                 {
120                     mx=mp[k][v];
121                     to[j][k]=v;
122                 }
123                 else if(mx==mp[k][v])
124                 {
125                     to[j][k]=min(v,to[j][k]);
126                 }
127             }
128         }
129     }
130     for(i=1;i<=n;i++)
131     {
132         addedge(i,i);
133     }
134     printf("%.3lf\n",F(s,t));
135     return 0;
136 }