平面上有 n 个点, 要求你画一点线, 将这 n 个点连接起来，任意两点间所画的线是直线．问你将这 n 个点连起来所画的线有多长．

用最小生成树搞搞就过了．

#include <bits/stdc++.h>
using namespace std;
const int N = 105;
struct Point {
    double x, y;
} p[N];
struct edge {
    int from, to;
    double dis;
    bool operator < (const edge &a) const {
        return dis < a.dis;
    }
} e[N * N];
int f[N];
int root(int x)
{
    return x == f[x] ? x : f[x] = root(f[x]);
}
double dis(Point a, Point b)
{
    return sqrt((a.x - b.x) * (a.x - b.x) + (a.y - b.y) * (a.y - b.y));
}
int main()
{
    int t;
    scanf("%d", &t);
    while (t--) {
        int n;
        scanf("%d", &n);
        for (int i = 0; i <= n; i++) {
            f[i] = i;
        }
        for (int i = 0; i < n; i++) {
            scanf("%lf %lf", &p[i].x, &p[i].y);
        }
        int cnt = 0;
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n; j++) {
                if (i == j)
                    continue;
                e[cnt++] = (edge){i, j, dis(p[i], p[j])};
            }
        }
        sort(e, e + cnt);
        double ans = 0;
        for (int i = 0; i < cnt; i++) {
            int x = root(e[i].from), y = root(e[i].to);
            if (x != y) {
                f[x] = y;
                ans += e[i].dis;
            }
        }
        printf("%.2lf\n", ans);
        if (t)
            puts("");
    }
    return 0;
}