牛的旅行

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
using namespace std;
const int maxn=157;
int x[maxn],y[maxn];
double map[maxn][maxn],dmax[maxn];
int n;
double d(int x,int y){
    double ret;
    ret=(double)sqrt(x*x+y*y);
    return ret;
}
int main(){
    cin>>n;
    memset(map,0x7f7f7f7f,sizeof(map));
    for(int i=1;i<=n;i++) map[i][i]=0;
    for(int i=1;i<=n;i++) cin>>x[i]>>y[i];
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            char a;cin>>a;
            if(a=='1'){
                map[i][j]=min(map[i][j],d(abs(x[i]-x[j]),abs(y[i]-y[j])));
                map[j][i]=map[i][j];
            }
        }
    }
    for(int k=1;k<=n;k++)
        for(int i=1;i<=n;i++)
            for(int j=1;j<=n;j++)
                if(map[i][j]>map[i][k]+map[k][j])
                    map[i][j]=map[i][k]+map[k][j]; 
    double l1=0;
    for(int i=1;i<=n;i++)
        for(int j=i+1;j<=n;j++)
            if(map[i][j]<0x7f7f7f7f){
                dmax[i]=max(dmax[i],map[i][j]);
                dmax[j]=max(dmax[j],map[i][j]);
                l1=max(l1,dmax[i]);l1=max(l1,dmax[j]);
            }
    int num1,num2;
    double ans=0x7f7f7f7f;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++){
            if(map[i][j]>=0x7f7f7f7f){
                if(ans>dmax[i]+dmax[j]+d(abs(x[i]-x[j]),abs(y[i]-y[j]))){
                    ans=dmax[i]+dmax[j]+d(abs(x[i]-x[j]),abs(y[i]-y[j]));
                    num1=i;num2=j;
                }
            }
        }
    }
    ans=max(ans,l1);
    printf("%.6lf
",ans);
    return 0;
} 

这种d[x]+d[y]+dis[x][y]的想法不是第一次见了，DP求树的直径时就是这样