次小生成树

// luogu-judger-enable-o2
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<cmath>
#include<iostream>
#define int long long 
using namespace std;
const int MAXN=400001;
const int INF=1e15+10;
inline int read()
{
    char c=getchar();int x=0,f=1;
    while(c<'0'||c>'9'){if(c=='-')f=-1;c=getchar();}
    while(c>='0'&&c<='9'){x=x*10+c-'0';c=getchar();}
    return x*f;
}
struct Edge
{
    int u,v,w;
}E[MAXN];
int Enum=1;
void Add(int x,int y,int z)
{
    E[Enum].u=x;
    E[Enum].v=y;
    E[Enum].w=z;Enum++;
}
struct node
{
    int u,v,w,nxt;
}edge[MAXN];
int head[MAXN];
int num=1;
int N,M;
int fa[MAXN],vis[MAXN],sum;
int deep[MAXN],f[MAXN][21],maxx[MAXN][21],minx[MAXN][21];
void AddEdge(int x,int y,int z)
{
    edge[num].u=x;
    edge[num].v=y;
    edge[num].w=z;
    edge[num].nxt=head[x];
    head[x]=num++;
}
int find(int x)
{
    if(fa[x]==x) return fa[x];
    else return fa[x]=find(fa[x]);
}
int unionn(int x,int y)
{
    int fx=find(x),fy=find(y);
    fa[fx]=fy;
}
int comp(const Edge &a,const Edge &b)
{
    return a.w<b.w;
}
void Kruskal()
{
    sort(E+1,E+Enum,comp);
    int tot=0;
    for(int i=1;i<=Enum-1;i++)
    {
        int x=E[i].u,y=E[i].v;
        if(find(x)!=find(y)) 
        {
            unionn(x,y),tot++,sum+=E[i].w,vis[i]=1;
            AddEdge(x,y,E[i].w);AddEdge(y,x,E[i].w);
        }
        if(tot==N-1) break;
    }
}
void dfs(int now,int fa)
{
    for(int i=head[now];i!=-1;i=edge[i].nxt)
    {
        if(edge[i].v==fa) continue;
        deep[edge[i].v]=deep[edge[i].u]+1;
        f[edge[i].v][0]=now;
        maxx[edge[i].v][0]=edge[i].w;
        dfs(edge[i].v,now);
    }
}
void pre()
{
    for(int i=1;i<=18;i++)
    {
        for(int j=1;j<=N;j++)
        {
            f[j][i]=f[ f[j][i-1] ][i-1];
            maxx[j][i]=max(maxx[j][i-1],maxx[ f[j][i-1] ][i-1]);
            minx[j][i]=max(minx[j][i-1],minx[ f[j][i-1] ][i-1]);
            if(maxx[j][i-1]>maxx[ f[j][i-1] ][i-1]) minx[j][i]=max(minx[j][i],maxx[ f[j][i-1] ][i-1]);
            else minx[j][i]=max(minx[j][i],maxx[j][i-1]);
        }
    }
}
int LCA(int x,int y)
{
    if(deep[x]<deep[y]) swap(x,y);
    for(int i=18;i>=0;i--)
        if(deep[ f[x][i] ] >= deep[y] ) 
            x=f[x][i];
    if(x==y) return x;
    for(int i=18;i>=0;i--)
        if(f[x][i] != f[y][i])
            x=f[x][i],y=f[y][i];
    return f[x][0];
}
int findmax(int x,int lca,int val)
{
    int ans=0;
    for(int i=18;i>=0;i--)
    {
        if(deep[ f[x][i] ] >= deep[lca]) 
        {
            if(maxx[x][i]==val) ans=max(ans,minx[x][i]);
            else ans=max(ans,maxx[x][i]);
            x=f[x][i];
        }
    }
    return ans;
}
void work()
{
    int ans=INF;
    for(int i=1;i<=Enum-1;i++)
    {
        if(vis[i]) continue;
        int x=E[i].u,y=E[i].v,z=E[i].w;
        int lca=LCA(x,y);
        int lmx=findmax(x,lca,z);
        int rmx=findmax(y,lca,z);
        if(max(lmx,rmx)!=z)
        ans=min(ans,sum+z-max(lmx,rmx));
    }
    printf("%lld",ans);
}
main()
{  
    #ifdef WIN32
    freopen("a.in","r",stdin);
    #else
    #endif
    N=read(),M=read();
    memset(head,-1,sizeof(head));
    for(int i=1;i<=N;i++) fa[i]=i;
    for(int i=1;i<=M;i++)
    {
        int x=read(),y=read(),z=read();
        Add(x,y,z);
    }
    Kruskal();
    deep[1]=1;
    dfs(1,0);
    pre();
    work();
    return 0;  
}