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 
    
     #define mp make_pairusing namespace std;typedef unsigned long long ull;typedef long long ll;template 
     
      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 
      
        > 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 
       
         > :: iterator it = Min.begin();            ans = min(ans, p[i].a - it -> first);        }    }    cout << ans << endl;    return 0;}