悬关，是一个状压思路的剪枝

#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int n;
bool flag[15];
double ans = 1e18, x[15], y[15], vis[70000][15];

inline double cal(double x, double y, double a, double b)
{
	return sqrt((x - a) * (x - a) + (y - b) * (y - b));
}

void dfs(int step, int last, int status, double sum)
{
	if (sum >= ans)
	{
		return;
	}
	if (step > n)
	{
		ans = min(ans, sum);
		return;
	}
	for (register int i = 1; i <= n; i++)
	{
		if (flag[i])
		{
			continue;
		}
		int st = status + (1 << (i - 1));
		double temp = cal(x[i], y[i], x[last], y[last]);
		if (vis[st][i] != 0 && vis[st][i] <= sum + temp)
		{
			continue;
		}
		flag[i] = true;
		vis[st][i] = sum + temp;
		dfs(step + 1, i, st, vis[st][i]);
		flag[i] = false;
	}
}
//15 0 0 1 1 1 -1 -1 1 -1 -1 2 2 2 0 2 -2 0 -2 -2 -2 -2 0 -2 2 0 2 1 3 1 4
int main()
{
	cin >> n;
	for (register int i = 1; i <= n; i++)
	{
		cin >> x[i] >> y[i];
	}
	for (register int i = 1; i < n; ++i)
		for (register int j = i + 1; j <= n; ++j)
			if (x[i] == x[j] && y[i] == y[j]) {
				for (register int k = j; k < n; ++k)
				{
					x[j] = x[k];
					y[j] = y[k];
				}
				--j;
				--n;
			}
	dfs(1, 0, 0, 0);
	cout << fixed << setprecision(2) << ans;
	return 0;
}