义乌中学暑假集训 2021.07.13 C

#include<bits/stdc++.h>
#define Tp template<typename Ty>
#define Ts template<typename Ty,typename... Ar>
#define W while
#define I inline
#define RI register int
#define int long long
#define Cn const
#define CI Cn int&
#define gc getchar
#define D isdigit(c=gc())
#define pc(c) putchar((c))
#define min(x,y) ((x)<(y)?(x):(y))
#define max(x,y) ((x)>(y)?(x):(y))
using namespace std;
#define eps 1e-7
namespace Debug{
    Tp I void _debug(Cn char* f,Ty t){cerr<<f<<'='<<t<<endl;}
    Ts I void _debug(Cn char* f,Ty x,Ar... y){W(*f!=',') cerr<<*f++;cerr<<'='<<x<<",";_debug(f+1,y...);}
    Tp ostream& operator<<(ostream& os,Cn vector<Ty>& V){os<<"[";for(Cn auto& vv:V) os<<vv<<",";os<<"]";return os;}
    #define gdb(...) _debug(#__VA_ARGS__,__VA_ARGS__)
}using namespace Debug;
namespace FastIO{
    Tp I void read(Ty& x){char c;int f=1;x=0;W(!D) f=c^'-'?1:-1;W(x=(x<<3)+(x<<1)+(c&15),D);x*=f;}
    Ts I void read(Ty& x,Ar&... y){read(x),read(y...);}
    Tp I void write(Ty x){x<0&&(pc('-'),x=-x,0),x<10?(pc(x+'0'),0):(write(x/10),pc(x%10+'0'),0);}
    Tp I void writeln(Cn Ty& x){write(x),pc('\n');}
}using namespace FastIO;
Cn int N=1e5+10,P=998244353;
int n,a[N],fir[N],nxt[N<<1],son[N<<1],tot,vis[N],Mx,ff;
double F[N],Ans=20000000000.0,g[N],b[N];
I void Add(CI x,CI y){nxt[++tot]=fir[x],fir[x]=tot,son[tot]=y;}
#define to son[i]
I void Dfs(CI x){RI i;for(F[x]=b[x],i=fir[x];i;i=nxt[i]) Dfs(to),F[x]+=max(0.0,F[to]);}
I void Dfs2(CI x){for(RI i=fir[x];i;i=nxt[i]) g[to]=max(0.0,g[x]-max(F[to],0.0))+F[to],Dfs2(to);}
I bool check(double x){
    RI i;for(i=1;i<=n;i++) b[i]=1.0*a[i]-x;ff=0;
    Dfs(1),g[1]=F[1],Dfs2(1);for(i=1;i<=n;i++) if(g[i]<0.0) return 1;
    return 0;
}
signed main(){
    RI i,j,x;double l=200000000.0,r=0.0,mid;for(read(n),i=1;i<=n;i++) read(a[i]),l=min(l,1.0*a[i]),r=max(r,1.0*a[i]);for(i=2;i<=n;i++) read(x),Add(x,i);
    W(r-l>eps) check(mid=(l+r)/2.0)?r=mid:l=mid;
    return printf("%.6lf\n",l),0;
}