次小生成树这题虽然A了但有一些疑问

1.下面代码中注释掉的部分为什么会死循环（自己机子上测的是，长时间不出答案
2.最后求最大/次大时的问题

code

#include <iostream>
#include <algorithm>
#define ls (o<<1)
#define rs (o<<1|1)
#define mid (s+((t-s)>>1))
#define int long long
#define mp (make_pair)
#define pii pair<int, int>
using namespace std;
const int maxn=1e6;
int ffa[maxn],fa[maxn],son[maxn],sz[maxn],de[maxn],tp[maxn],dfn[maxn],va[maxn],hd[maxn],cst[maxn];
int sztree,cntnode;
int n, m,u,v,vale,cnt,num,ecnt;
bool vis[maxn];
pair<int, int> sumv[maxn];
template <typename T>
void read(T &x)
{
   x = 0;
   int f = 1;
   char c = getchar();
   for (; !isdigit(c); c = getchar())
       if (c == '-')
           f = -1;
   for (; isdigit(c); c = getchar())
       x = (x << 1) + (x << 3) + c - '0';
   x *= f;
}
struct node1{
   int from, to, val;
   bool operator < (const node1 b) const{
   	return val < b.val;
   }
}edge[maxn];
struct node2{
   int to, nex, val;
}e[maxn];
inline void adde(int from, int to, int val)
{
   e[++cnt] = node2{to, hd[from], val};
   hd[from] = cnt;
}

inline int find(int x) { return x==ffa[x] ? x : ffa[x]=find(ffa[x]);}
inline void merge(int x, int y)
{
   x=find(x), y=find(y);
   ffa[x] = y;
}
void mst()
{
   sort(edge+1, edge+1+m);
   for (int i=1; i<=m; i++)
   {
   	u = edge[i].from, v=edge[i].to, vale=edge[i].val;
   	if (find(u) == find (v)) continue;
   	vis[i] = true;	
   	merge(u, v);
   	sztree += vale;
   	adde(u,v,vale),adde(v,u,vale);
   	if (++cntnode == n-1) break;
   }
}

inline void dfs1(int nw, int en)
{
   fa[nw] = en;
   sz[nw] = 1;
   de[nw] = de[en] + 1;
   for (int i=hd[nw]; i; i=e[i].nex)
   {
   	if (e[i].to==en)
   		continue;
   	va[e[i].to] = e[i].val;
   	dfs1(e[i].to, nw);
   	sz[nw] += sz[e[i].to];
   	if (sz[e[i].to] > sz[son[nw]])
   		son[nw] = e[i].to;
   }
}

inline void dfs2(int nw, int en)
{
   tp[nw] = en;
   dfn[nw] = ++num;
   cst[num] = va[nw];
   if (son[nw]) dfs2(son[nw], en);
   for (int i=hd[nw]; i; i=e[i].nex)
   {
   	if (e[i].to==fa[nw] || e[i].to == son[nw])
   		continue;
   	dfs2(e[i].to, e[i].to);
   }
}

inline pii getmax(pii x, pii y)
{
   pii ss;
   if (x.first == y.first){
   	ss.first=x.first;
   	ss.second=max(x.second, y.second);
   }
   else 
   {
   	ss.first=max(x.first, y.first);
   	ss.second=max(min(x.first, y.first), max(x.second, y.second));
   }
   return ss;
}

inline void push_up(int o)
{
   sumv[o] = getmax(sumv[ls], sumv[rs]);
}

inline void build(int o, int s, int t)
{
   if (s==t) {
   	sumv[o].first = cst[s];
   	return ;
   }
   build(ls, s, mid);
   build(rs, mid+1, t);
   push_up(o);
}

inline pii query(int o, int l, int r, int s, int t)
{
   if (l<=s && r>=t){
   	return sumv[o];
   }
   pii ss(0,0);
   if (l <= mid) ss=query(ls, l, r, s, mid);
   if (r > mid) ss=getmax(ss, query(rs, l, r, mid+1, t));
   return ss;
   
   
 ------ here is Q1---------------------------------
   /*   
   if (r <= mid) return query(ls, l, r, s, mid);
   if (l > mid) return query(rs, l, r, mid+1, t);
   return getmax(query(ls, l, r, s, mid), query(rs, l, r, mid+1, t));
   */
  -------------------------------------------------
   
   
}

inline pii lca(int a, int b)
{
   pii ss(0,0);
   while (tp[a] != tp[b])
   {
   	if (de[tp[a]] < de[tp[b]])
   		swap(a,b);
   	ss=getmax(ss, query(1, dfn[tp[a]], dfn[a], 1, n));
   	a=fa[tp[a]];
   }
   if (de[a] > de[b])	
   	swap(a, b);
  ------here is Q2---------------------------------------
   ss=getmax(ss, query(1, dfn[a]+1, dfn[b], 1, n));
  //为什么会是dfn[a]+1而不是dfn[a]
  -------------------------------------------------------
   return ss;
} 

void init()
{
   mst();
   dfs1(1, 0);
   dfs2(1, 1);
   build(1, 1, n);
}

signed main()
{
   freopen("C://Users//19637//Downloads//tree4.in", "r", stdin);
   //ios::sync_with_stdio(false);
   read(n),read(m);
   for (int i=1; i<=m; i++)
   {
   	ffa[i] = i;
   	read(u),read(v),read(vale);
   	if (u==v) continue;
   	edge[++ecnt] = {u, v, vale};
   }
   m = ecnt;
   init();
   int dea=1e16;
   for (int i=1; i<=m; i++)
   {
   	if (vis[i]) continue;
   	u=edge[i].from, v=edge[i].to, vale=edge[i].val;
   	pii ss=lca(u, v);
   	if (ss.first < vale)
   		dea = min(dea, vale-ss.first);
   	else if (ss.second)
   		dea = min(dea, vale-ss.second);
   	//if(dea==0) cout << u << "---------------" << v << endl;
   }
   //cout << "mst=" <<  sztree << endl;
   cout << sztree+dea;
   return 0;

}