时间复杂度:mlogm

流程：
1.建立并查集.
2.边按权值排序.
3.若x,y同属一个集合，则忽略.
4.否则，将x,y合并，并累加z.
5.处理过的边就是最小生成树.

struct rec
{
    int x,y,z;
}edge[N];
int ans,n,m;
int fa[N];

bool operator < (rec a,rec b)
{
    return a.z < b.z;
}

int get(int x)
{
    if(x == fa[x]) return x;
    return fa[x] = get(fa[x));
}

int main()
{
    cin>>n>>m;
    for(int i = 1; i <= m; i ++ )
        cin>>edge[i].x>>edge[i].y>>edge[i].z;

    for(int i = 1; i <= n; i ++ )
        fa[i] = i;

    sort(edge+1,edge+1+m);

    for(int i = 1; i <= m; i ++ )
    {
        int x = get(edge[i].x);
        int y = get(edge[i].y);
        if(x == y) continue;
        fa[x] = y;
        ans+=edge[i].z;
    }
    cout<<ans<<endl;
}