SHOI2014 概率充电器

Posted 2021-12-01 fengxunling

题目链接：戳我

非常不好意思，因为想要排版，所以今天先只把代码贴出来，明天补题解。QAQ

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#define eps 1e-7
#define MAXN 500010
int n,m,t;
int head[MAXN],fa[MAXN];
double h[MAXN];
struct Edge{int nxt,to,dis;}edge[MAXN<<1];
inline void add(int from,int to,int dis)
{
    edge[++t].nxt=head[from],edge[t].to=to,edge[t].dis=dis;
    head[from]=t;
}
inline void dfs1(int x,int pre)
{
    fa[x]=pre;
    for(int i=head[x];i;i=edge[i].nxt)
    {
        int v=edge[i].to;
        if(v==pre) continue;
        dfs1(v,x);
        double k=h[v]*(1.0*edge[i].dis/100);
        h[x]+=k-k*h[x];
    }
}
inline void dfs2(int x,int pre)
{
    for(int i=head[x];i;i=edge[i].nxt)
    {
        int v=edge[i].to;
        if(v==fa[x]) continue;
        double cur=h[v]*(1.0*edge[i].dis/100);
        if(fabs(cur-1.0)>eps) 
        {
            double k=(h[x]-cur)/(1-cur);
            k*=1.0*edge[i].dis/100;
            h[v]+=k-k*h[v];
        }
        dfs2(v,x);
    }
}
int main()
{
    #ifndef ONLINE_JUDGE
    freopen("ce.in","r",stdin);
    #endif
    scanf("%d",&n);
    for(int i=1;i<=n-1;i++)
    {
        int u,v,w;
        scanf("%d%d%d",&u,&v,&w);
        add(u,v,w),add(v,u,w);
    }
    for(int i=1;i<=n;i++)
    {
        int x;
        scanf("%d",&x);
        h[i]=1.0*x/100;
    }
    dfs1(1,0);
    dfs2(1,0);
    double ans=0;
    for(int i=1;i<=n;i++) ans+=h[i];
    printf("%.6lf\n",ans);
    return 0;
}