codeforces679 D. Bear and Chase

codeforces679 D. Bear and Chase
http://blog.csdn.net/Ezereal/article/details/52743686
D Bear and Chase

O(n^3) 暴力
首先 ，枚举第一次放BCD地方
然后枚举罪犯离BCD的距离
然后枚举一下第二次方BCD的地方
再加一点预处理就行了

#include<cstdio>
#include<cstring>
using namespace std;
const int N=440;
const double eps=1e-9;
int i,j,k,l,n,m,ch,En,x,y;
int h[N],du[N],a[N][N],b[N][N],nn[N];
double ans,now,p[N],q[N];
struct cc { double p;int x;} A[N];
struct edge { int s,n;} E[N*N];
void R(int &x) {
    x=0;ch=getchar();
    while (ch<'0' || '9'<ch) ch=getchar();
    while ('0'<=ch && ch<='9') x=x*10+ch-'0',ch=getchar();
}
void E_add(int x,int y) {
    du[x]++;du[y]++;
    a[x][y]=a[y][x]=1;
    E[++En].s=y;E[En].n=h[x];h[x]=En;
    E[++En].s=x;E[En].n=h[y];h[y]=En;
}
double max(double x,double y) {
    if (x>y) return x;
    return y;
}
double get_num(int x) {
    int i,j,k,nm=0;
    for (i=1;i<=n;i++) p[i]=0.0;
    for (i=1;i<=nn[x];i++)
        for (k=h[b[x][i]];k;k=E[k].n) p[E[k].s]+=1.0/(1.0*n*du[b[x][i]]);
    for (i=1;i<=n;i++) if (p[i]>eps) {
        A[++nm].p=p[i];
        A[nm].x=i;
    }
    double ans=0.0,now;
    for (i=1;i<=n;i++) {
        now=0.0;
        for (j=1;j<=nm;j++) q[a[i][A[j].x]]=0.0;
        for (j=1;j<=nm;j++) if (A[j].p>q[a[i][A[j].x]]) {
            now+=A[j].p-q[a[i][A[j].x]];
            q[a[i][A[j].x]]=A[j].p;
        }
        ans=max(ans,now);
    }
    return ans;
}
int main() {
    memset(a,60,sizeof a);
    R(n);R(m);
    for (i=1;i<=n;i++) a[i][i]=0; 
    for (i=1;i<=m;i++) {
        R(x);R(y);
        E_add(x,y);
    }
    for (k=1;k<=n;k++)
        for (i=1;i<=n;i++)
            for (j=1;j<=n;j++) if (a[i][k]+a[k][j]<a[i][j]) a[i][j]=a[i][k]+a[k][j];
    for (i=1;i<=n;i++) {
        now=0.0;
        for (j=0;j<n;j++) nn[j]=0;
        for (j=1;j<=n;j++) {
            k=a[i][j];
            b[k][++nn[k]]=j;
        }
        for (j=0;j<n;j++) if (nn[j]) {
            if (nn[j]==1) now+=1.0/(1.0*n);
            else now+=max(1.0/(1.0*n),get_num(j));
        }
        ans=max(ans,now);
    }
    printf("%.9lf
",ans);
}