傳送門：http://poj.org/problem?id=2420

【題解】

費馬點問題，模擬退火。%.0f神坑

# include <math.h>
# include <stdio.h>
# include <stdlib.h>
# include <string.h>
# include <iostream>
# include <algorithm>
// # include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef 
 
long double ld;
typedef unsigned long long ull;
const int M = 5e5 + 10;
const int mod = 1e9+7;
const double pi = acos(-1.0);

# define RG register
# define ST static

double minx, maxx, miny, maxy;
int n;
struct pa {
    double x, y;
    double dis;
    pa() {}
    pa(double x, double y, double dis) : x(x), y(y), dis(dis) {}
}a[M];

 
namespace SA {
    const double eps = 1e-2, DEC = 0.9, ACCEPT_DEC = 0.5;
    const int N = 30, T = 30, RAD = 1000; 
    inline double rand01() {
        return rand() % (RAD + 1) / (1.0 * RAD);
    }
    inline double getdist(double x, double y) {
        double ret = 0;
        for (int i=1; i<=n; ++i)
            ret 
 
+= sqrt((x-a[i].x)*(x-a[i].x)+(y-a[i].y)*(y-a[i].y));
        return ret;
    }
    inline pa randpoint(double px, double py, double qx, double qy) {
        double x = (qx - px) * rand01() + px, y = (qy - py) * rand01() + py;
        return pa(x, y, getdist(x, y)); 
    }
    pa res[N + 5];
    inline double main() {
        res[1] = pa((minx+maxx)/2, (miny+maxy)/2, getdist((minx+maxx)/2, (miny+maxy)/2));
        for (int i=2; i<=N; ++i) {
            double x = rand01() * (maxx - minx) + minx;
            double y = rand01() * (maxy - miny) + miny;
            res[i] = pa(x, y, getdist(x, y));
        }
        double temper = (maxx+maxy-minx-miny)*n, accept = 0.6;
        while(temper > eps) {
            for (int i=1; i<=N; ++i)
                for (int j=1; j<=T; ++j) {
                    pa t = randpoint(max(res[i].x - temper, minx), max(res[i].y - temper, miny), min(res[i].x + temper, maxx), min(res[i].y + temper, maxy));
                    if(t.dis < res[i].dis) res[i] = t;
                    else if(rand01() <= accept) res[i] = t;
                }
            temper *= DEC;
            accept *= ACCEPT_DEC;
        }
        double dis = 1e18;
        for (int i=1; i<=N; ++i) 
            if(res[i].dis < dis) dis = res[i].dis;
        return dis;
    }
}

int main() {
    srand(19260817); 
    while(cin >> n) {
        maxx = maxy = -1e18, minx = miny = 1e18;
        for (int i=1; i<=n; ++i) {
            scanf("%lf%lf", &a[i].x, &a[i].y);
            maxx = max(maxx, a[i].x);
            minx = min(minx, a[i].x);
            maxy = max(maxy, a[i].y);
            miny = min(miny, a[i].y);
        }
        double ans = SA::main();
        printf("%.0f\n", ans);
    }

    return 0;
}

poj2420 A Star not a Tree?