https://www.luogu.org/problemnew/show/P4234

按照边的权值从小到大排序，依次加入，并删除能够删除的权值最小的一条边，用 set 维护当前所有边的边权，并查集维护联通性，LCT 维护两点间最小值和 link cut 操作即可

#include <bits/stdc++.h>
#define mp make_pair
using namespace std;typedef unsigned long long ull;
typedef long long ll;template <typename _T>
inline void read(_T &f) {f = 0; _T fu = 1; char c = getchar();while(c < '0' || c > '9') {if(c == '-') fu = -1; c = getchar();}while(c >= '0' && c <= '9') {f = (f << 3) + (f << 1) + (c & 15); c = getchar();}f *= fu;
}const int N = 250000 + 10;struct ele {int u, v, a;bool operator < (const ele A) const {return a < A.a;}
}p[N];set < pair <int, int> > Min;
int fa[N], ch[N][2], rev[N], minn[N], val[N], wz[N], st[N], f[N], n, m, len, cnt, ans = INT_MAX;int isroot(int u) {return ch[fa[u]][0] != u && ch[fa[u]][1] != u;}
int get(int u) {return ch[fa[u]][1] == u;}void update(int u) {minn[u] = val[u]; wz[u] = u;if(minn[ch[u][0]] < minn[u] && ch[u][0]) minn[u] = minn[ch[u][0]], wz[u] = wz[ch[u][0]];if(minn[ch[u][1]] < minn[u] && ch[u][1]) minn[u] = minn[ch[u][1]], wz[u] = wz[ch[u][1]];
}void pushdown(int u) {if(rev[u]) {swap(ch[u][0], ch[u][1]);rev[ch[u][0]] ^= 1;rev[ch[u][1]] ^= 1;rev[u] ^= 1;}
}void rotate(int u) {int old = fa[u], oldd = fa[old], k = get(u);if(!isroot(old)) ch[oldd][get(old)] = u; fa[u] = oldd;ch[old][k] = ch[u][k ^ 1]; fa[ch[u][k ^ 1]] = old;fa[old] = u; ch[u][k ^ 1] = old;update(old); update(u);
}void splay(int u) {st[len = 1] = u;for(int i = u; !isroot(i); i = fa[i]) st[++len] = fa[i];for(int i = len; i >= 1; i--) pushdown(st[i]);for(; !isroot(u); rotate(u)) if(!isroot(fa[u])) rotate(get(u) == get(fa[u]) ? fa[u] : u);
}void access(int u) {for(int i = 0; u; i = u, u = fa[u]) {splay(u);ch[u][1] = i;update(u);}
}void makeroot(int u) {access(u);splay(u);rev[u] ^= 1;
}void link(int u, int v) {makeroot(u);fa[u] = v;
}void cut(int u, int v) {makeroot(u);access(v);splay(v);fa[u] = ch[v][0] = 0;update(v);
}int query(int u, int v) {makeroot(u);access(v);splay(v);return wz[v];
}int find(int x) {return f[x] == x ? x : f[x] = find(f[x]);}int main() {read(n); read(m);for(int i = 1; i <= n; i++) f[i] = i, val[i] = INT_MAX;for(int i = 1; i <= m; i++) {read(p[i].u);read(p[i].v);read(p[i].a);}sort(p + 1, p + m + 1);for(int i = 1; i <= m; i++) {int x = p[i].u, y = p[i].v;if(find(x) != find(y)) {cnt++; f[find(x)] = find(y);val[i + n] = p[i].a;link(i + n, x);link(i + n, y);Min.insert(mp(p[i].a, i)); } else {if(x == y) continue;int wz = query(x, y);val[i + n] = p[i].a;cut(wz, p[wz - n].u);cut(wz, p[wz - n].v);link(i + n, x);link(i + n, y);Min.erase(mp(p[wz - n].a, wz - n));Min.insert(mp(p[i].a, i));}if(cnt == n - 1) {set < pair <int, int> > :: iterator it = Min.begin();ans = min(ans, p[i].a - it -> first);}}cout << ans << endl;return 0;
}