O(mlogm)

int p[N]; //并查集

struct Edge
{
    int a, b, w;
    bool operator<(const Edge &W) const
    {
        return w < W.w;
    }
} edge[M];

int find(int x) //并查集
{
    if (p[x] != x)
    {
        p[x] = find(p[x]);
    }
    return p[x];
}

int kruskal() //连通时返回最小生成树边权和，否则返回INF(0x3f3f3f3f)
{
    sort(edge, edge + m);
    for (int i = 1; i <= n; i++)
    {
        p[i] = i;
    }
    int res = 0, cnt = 0;
    for (int i = 0; i < m; i++)
    {
        int a = find(edge[i].a), b = find(edge[i].b);
        if (a != b)
        {
            p[a] = b;
            res += edge[i].w;
            cnt++;
        }
    }
    if (cnt < n - 1)
    {
        return INF;
    }
    return res;
}