kruskal：

将图中的边按边权排好升序，然后从小的开始遍历，如果这条边的起点终点在一个联通图（并查集）里就不加入这条边，反之加入边，并且两个点加到一个并查集里。

 

#include<cstdio>
#include<algorithm>
#define MAX_N 111
using namespace std;
int N, M;
struct edge {
	int from, to;
	int cost;
}E[MAX_N*MAX_N];

int father[MAX_N];
bool cmp(edge a, edge b)
{
	return a.cost < b.cost;
}

void init() {
	for (int i = 1;i < MAX_N;i++)
		father[i] = i;
}
int  fid(int x)
{
	if (x == father[x])
		return x;
	return father[x] = fid(father[x]);
}
bool same(int x, int y)
{
	return fid(x) == fid(y);
}

void unionset(int x, int y)
{
	int u = fid(x), v = fid(y);
	if (u == v)
		return;
	father[u] = v;
}

long long kruskal()
{
	long long res=0;
	sort(E + 1,E + 1 + M, cmp);
	for (int i = 1;i <= M;i++)
	{
		if (same(E[i].from, E[i].to))
			continue;
		unionset(E[i].from, E[i].to);
		res += E[i].cost;
	}
	return res;
}

int main()
{
	while (scanf("%d%d", &M, &N) == 2)
	{
		if (M == 0) break;
		init();
		for (int i = 1;i <= M;i++)
			scanf("%d%d%lld", &E[i].from, &E[i].to, &E[i].cost);
		int res = kruskal();
		for (int i = 1;i <= N;i++)
			if (!same(i, 1))
				res = -1;
		if (res == -1)
			printf("?\n");
		else
			printf("%lld\n", res);

	}
	return 0;
}