非严格次小生成树

很简单，先做最小生成树

然后枚举没加入的边加入，替换掉这个环内最大的边

最后取\(min\)

严格次小生成树

还是一样的

可以考虑维护一个严格次大值

最大值和枚举的边相同就替换次大值的边

否则替换最大值的边

最后取\(min\)

裸题

Luogu

随你用各种姿势\(AC\)

\(LCT\)常数大，但是好写，开\(O2\)可以过

# include <bits/stdc++.h>
# define RG register
# define IL inline
# define Fill(a, b) memset(a, b, sizeof(a))
using namespace std;
typedef long long ll;
const int _(4e5 + 5);IL ll Input(){
    RG ll x = 0, z = 1; RG char c = getchar();
    for(; c < '0' || c > '9'; c = getchar()) z = c == '-' ? -1 : 1;
    for(; c >= '0' && c <= '9'; c = getchar()) x = (x << 1) + (x << 3) + (c ^ 48);
    return x * z;
}int ch[2][_], fa[_], rev[_], S[_], mx[_], _mx[_], val[_], anc[_];
int n, m, ff[_], tt[_], w[_], id[_], vis[_];
ll ans = 1e18, mst;# define ls ch[0][x]
# define rs ch[1][x]IL int Son(RG int x){  return ch[1][fa[x]] == x;  }IL int Isroot(RG int x){  return ch[0][fa[x]] != x && ch[1][fa[x]] != x;  }IL void Update(RG int x){
    mx[x] = val[x], _mx[x] = -1;
if(mx[ls] > mx[x]) _mx[x] = mx[x], mx[x] = mx[ls];
else _mx[x] = max(mx[ls], _mx[x]);
if(mx[rs] > mx[x]) _mx[x] = mx[x], mx[x] = mx[rs];
else _mx[x] = max(mx[rs], _mx[x]);
_mx[x] = max(_mx[x], max(_mx[ls], _mx[rs]));
}IL void Reverse(RG int x){
if(!x) return;
rev[x] ^= 1, swap(ls, rs);
}IL void Pushdown(RG int x){
if(!rev[x]) return;
rev[x] = 0, Reverse(ls), Reverse(rs);
}IL void Rotate(RG int x){
    RG int y = fa[x], z = fa[y], c = Son(x);
    if(!Isroot(y)) ch[Son(y)][z] = x; fa[x] = z;
    ch[c][y] = ch[!c][x]; fa[ch[c][y]] = y;
    ch[!c][x] = y; fa[y] = x;
Update(y);
}IL void Splay(RG int x){
    RG int top = 0; S[++top] = x;
    for(RG int y = x; !Isroot(y); y = fa[y]) S[++top] = fa[y];
    while(top) Pushdown(S[top--]);
    for(RG int y = fa[x]; !Isroot(x); Rotate(x), y = fa[x])
        if(!Isroot(y)) Son(x) ^ Son(y) ? Rotate(x) : Rotate(y);
    Update(x);
}IL void Access(RG int x){
for(RG int y = 0; x; y = x, x = fa[x]) Splay(x), rs = y, Update(x);
}IL void Makeroot(RG int x){
Access(x), Splay(x), Reverse(x);
}IL int Find(RG int x){
return anc[x] == x ? x : anc[x] = Find(anc[x]);
}IL void Split(RG int x, RG int y){
Makeroot(x), Access(y), Splay(y);
}IL void Link(RG int x, RG int y){
Makeroot(x), fa[x] = y;
}IL int Cmp(RG int x, RG int y){  return w[x] < w[y];  }int main(RG int argc, RG char* argv[]){
    n = Input(), m = Input();
for(RG int i = 1; i <= n; ++i) val[i] = -1, anc[i] = i;
for(RG int i = 1; i <= m; ++i)
ff[i] = Input(), tt[i] = Input(), val[n + i] = w[i] = Input(), id[i] = i;
sort(id + 1, id + m + 1, Cmp);
for(RG int i = 1, j = 1; i <= m && j < n; ++i){
RG int fx = Find(ff[id[i]]), fy = Find(tt[id[i]]);
if(fx == fy) continue;
anc[fx] = fy, mst += w[id[i]], ++j, vis[id[i]] = 1;
Link(ff[id[i]], id[i] + n), Link(tt[id[i]], id[i] + n);
}
for(RG int i = 1; i <= m; ++i){
if(vis[i]) continue;
Split(ff[i], tt[i]);
if(mx[tt[i]] != w[i]) ans = min(ans, mst - mx[tt[i]] + w[i]);
else if(_mx[tt[i]] != w[i]) ans = min(ans, mst - _mx[tt[i]] + w[i]);
}
printf("%lld\n", ans);
    return 0;
}