bzoj4726 [POI2017]Sabota 二分+暴力

直接二分，判断的话太显然了吧。。
不知道为什么那么少人A。

#include<cstdio>#include<algorithm>#include<cstring>#include<cmath>#define fo(i,a,b) for(int i=a;i<=b;i++)#define fd(i,a,b) for(int i=a;i>=b;i--)using namespace std;const double eps=1e-7;const int N=5e5+5;int siz[N],fa[N],tot,head[N],go[N],next[N];int mx[N];int n,k;inline bool pd(double x){ fo(i,1,n)mx[i] = 0;    fd(i, n, 1){        if(siz[i]) if(mx[i] / (double)siz[i] - x > eps) mx[i] = siz[i] + 1;        mx[i] = max(mx[i], 1);        mx[fa[i]] = max(mx[fa[i]], mx[i]);    }    if(mx[1] <= k) return true;    return false;}int main(){    scanf("%d%d",&n,&k);    fo(i,2,n)    {        int x;        scanf("%d",&x);        fa[i]=x;    }    fo(i,1,n)siz[i]=1;    fd(i,n,1)siz[fa[i]]+=siz[i];    fo(i,1,n)siz[i]--;    double l = 0.0, r = 1.0, ans,mid;    while(abs(r - l) > eps){         mid = (l + r) / 2.0;        if(pd(mid)) r = mid;        else l = mid;    }    printf("%.9lf\n",r);}