http://poj.org/problem?id=1287
#include<iostream>
#include<math.h>
#include<stdio.h>
#include<string.h>
#include<math.h>
#include<algorithm>
#define maxn 106
#define INF 1000000
using namespace std;
int n,m;
int graph[maxn][maxn];
int lowcost[maxn];
int visited[maxn];
void createGraph()
{
memset(graph,INF,sizeof(graph));
memset(lowcost, 0, sizeof(lowcost));
memset(visited, 0, sizeof(visited));
int u,v,w;
for(int i=1;i<=m;i++){
cin>>u>>v>>w;
if(graph[u][v]>w)
{
graph[u][v]=graph[v][u]=w;
}//Prim 算法需要进行判断最小重边
}
}
void prim()
{
int sum=0,cnt=0;
visited[1]=1;
for(int i=1;i<=n;i++){
lowcost[i]=graph[1][i];
}
while(true)
{
int minn=lowcost[1];
int index=1;
for(int j=2;j<=n;j++){
if(!visited[j]&&lowcost[j]<minn){
minn=lowcost[j];
index=j;
}
}
if(index==1)break;
sum+=minn;
cnt++;
visited[index]=1;
for(int j=1;j<=n;j++){
if(!visited[j]&&lowcost[j]>graph[index][j])
{
lowcost[j]=graph[index][j];
}
}
}
if(cnt==n-1)cout<<sum<<endl;
else cout<<"?"<<endl;
}
int main()
{
while(cin>>n>>m&&n)
{
createGraph();
prim();
}
return 0;
}