时间复杂度:mlogn

void prim()
{
    memset(d,0x3f,sizeof d);
    memset(v,0,sizeof v);
    d[1] = 0;
    for(int i = 1; i < n; i ++ )
    {
        int x = 0;
        for(int j = 1; j <= n; j ++ )
            if(!v[j] && (x == 0||d[j] < d[x] ) x = j;
        v[x] = 1;
        for(int y = 1; y <= n; y ++ )
            if(!v[y]) d[y] = min(d[y],a[x][y]);
    }

}