hdu4126 MST修改边后的期望值 树形dp 好题

#pragma comment(linker, "/STACK:102400000,102400000")
#include <iostream>
#include<stdio.h>
#include<string.h>
#include<vector>
#include<algorithm>
#include<cmath>
#include<map>
#include<queue>
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1 | 1
#define lowbit(x) x&(-x)
template <class T>
inline bool rd(T &ret){
    char c;int sgn;
    if(c=getchar(),c==EOF) return 0;
    while(c!='-' && (c<'0' || c>'9')) c=getchar();
    sgn=(c=='-')?-1:1;
    ret=(c=='-')?0:(c-'0');
    while(c=getchar(),c>='0' && c<='9') ret=ret*10+(c-'0');
    ret*=sgn;
    return 1;
}
template <class T>
inline void pt(T x){
        if(x<0){
            putchar('-');
            x=-x;
        }
        if(x>9) pt(x/10);
        putchar(x%10+'0');
}
using namespace std;
typedef long long ll;
const int N=3005;
long long inf=1e15;
const int MOD=1e9+7;
int T,n,m,k,x,y,z,l,tot,cnt;
ll g[N][N],d[N],mst,cost[N][N];
bool vis[N],choose[N][N];
vector<int> G[N];
int pre[N];
ll dfs(int u,int fa,int src){/*返回src为根的树中，src到u子树的非树边的最短距离*/
    ll siz=inf;
    for(int i=0;i<G[u].size();i++){
        int v=G[u][i];
        if(v==fa) continue;
        ll tmp=dfs(v,u,src);
        siz=min(siz,tmp);
        cost[u][v]=min(cost[u][v],tmp);
    }
    if(fa!=src)/*如果不是树边*/
        siz=min(siz,g[u][src]);
    return siz;
}
void MST(){
    for(int i=0;i<n;i++)
    for(int j=0;j<n;j++)
        cost[i][j]=g[i][j]=inf,choose[i][j]=0;
    while(m--){
        int u,v;ll dis;rd(u);rd(v);rd(dis);
        g[u][v]=g[v][u]=min(g[u][v],dis);
    }
    for(int i=0;i<n;i++){
        d[i]=inf;
        G[i].clear();
        vis[i]=0;
        pre[i]=-1;
    }

    d[0]=0;
    mst=0;
    for(int i=0;i<n;i++){
        int pos=-1;
        for(int j=0;j<n;j++)
            if(!vis[j] && (pos==-1 || d[pos]>d[j]))
                pos=j;
        if(pre[pos]!=-1){
            G[pos].push_back(pre[pos]);
            G[pre[pos]].push_back(pos);
            choose[pos][pre[pos]]=choose[pre[pos]][pos]=1;
        }
        for(int j=0;j<n;j++)
            if(d[j]>g[j][pos]){
                d[j]=g[j][pos];
                pre[j]=pos;
            }
        vis[pos]=1;
        mst+=d[pos];
    }
}
int main()
{
#ifndef  ONLINE_JUDGE
 freopen("aaa","r",stdin);
#endif
   int q,u,v;ll dis;
   while(~scanf("%d%d",&n,&m),m+n){
          MST();
          for(int i=0;i<n;i++) dfs(i,-1,i);
          rd(q);
          ll ans=0;
          for(int i=1;i<=q;i++){
            rd(u);rd(v);rd(dis);
            if(choose[u][v]==false) ans+=mst;/*修改边不在最小生成树上*/
            else ans+=mst-g[u][v]+min(cost[u][v],dis);/*修改边在最小生成树上*/
          }
          printf("%.4f\n",(double)ans/(double)q);

   }
    return 0;
}