一直WA了两个点，和答案很接近了...但是代码太长好难调啊...

#include<bits/stdc++.h>
#define ll long long
using namespace std;

int N, M, a[100010];
int cnt, head[100010];
int LN, dep[100010], f[100010][25];
bool used[300010];
ll sum, ans = 2e15;

struct pre_edge{
	int x, y;
	ll z;
}pe[300010];

struct edge{
	int to, next;
	ll val;
}e[600010];

struct M4{
	ll m[5] = {-1e9, -1e9, -1e9, -1e9, -1e9}, m1, m2 = -1e9;
	void msort(){
		sort(m + 1, m + 5);
		m1 = m[4];
		for(int i = 4; i >= 0; i--){
			if(m[i] < m1){
				m2 = m[i];
				return;
			}
		}
	}
}maxx[100010][25];

void add_edge(int u, int v, ll w){
	e[++cnt].to = v;
	e[cnt].val = w;
	e[cnt].next = head[u];
	head[u] = cnt;
}

bool cmp(pre_edge a, pre_edge b){
	return a.z < b.z;
}

int find(int x){
	while(x != a[x]){
		x = a[x] = a[a[x]];
	}
	return x;
}

void kruskal(){
	sort(pe + 1, pe + M + 1, cmp);
	int i, E = 0;
	for(i = 1; i <= M; i++){
		int ax = find(pe[i].x), ay = find(pe[i].y);
		if(ax == ay){
			continue;
		}
		sum += pe[i].z;
		used[i] = 1;
		a[ax] = ay;
		add_edge(pe[i].x, pe[i].y, pe[i].z);
		add_edge(pe[i].y, pe[i].x, pe[i].z);
		if(++E == N - 1){
			return;
		}
	}
}

void init(int u, int fa, ll val){
	dep[u] = dep[fa] + 1;
	for(int i = 1; i <= 4; i++){
		maxx[u][0].m[i] = val;
	}
	maxx[u][0].msort();
	for(int i = 0; i < LN; i++){
		f[u][i + 1] = f[f[u][i]][i];
		maxx[u][i + 1].m[1] = maxx[u][i].m1;
		maxx[u][i + 1].m[2] = maxx[u][i].m2;
		maxx[u][i + 1].m[3] = maxx[f[u][i]][i].m1;
		maxx[u][i + 1].m[4] = maxx[f[u][i]][i].m2;
		maxx[u][i + 1].msort();
	}
	for(int i = head[u]; i; i = e[i].next){
		int v = e[i].to;
		if(v == fa){
			continue;
		}
		f[v][0] = u;
		init(v, u, e[i].val);
	}
}

ll LCA(int x, int y, int dis){
	ll xm1 = -1e9, ym1 = -1e9, xm2 = -1e9, ym2 = -1e9;
	if(dep[x] < dep[y]){
		swap(x, y);
	}
	for(int i = LN; i >= 0; i--){
		if(dep[f[x][i]] > dep[y]){
			xm1 = max(xm1, maxx[x][i].m1);
			xm2 = max(xm2, maxx[x][i].m2);
			x = f[x][i];
		}
		if(x == y){
			return (dis > xm1 ? xm1 : xm2);
		}
	}
	for(int i = LN; i >= 0; i--){
		if(f[x][i] != f[y][i]){
			xm1 = max(xm1, maxx[x][i].m1);
			xm2 = max(xm2, maxx[x][i].m2);
			ym1 = max(ym1, maxx[y][i].m1);
			ym2 = max(ym2, maxx[y][i].m2);
			x = f[x][i];
			y = f[y][i];
		}
	}
	xm1 = max(xm1, maxx[x][0].m1);
	xm2 = max(xm2, maxx[x][0].m2);
	ym1 = max(ym1, maxx[y][0].m1);
	ym2 = max(ym2, maxx[y][0].m2);
	ll t[5] = {-1e9, xm1, xm2, ym1, ym2};
	sort(t + 1, t + 5);
	for(int i = 4; i >= 0; i--){
		if(t[i] < dis){
			return t[i];
		}
	}
}

int main(){
	freopen("tree4.in", "r", stdin);
	scanf("%d %d", &N, &M);
	LN = log2(N) + 1;
	for(int i = 1; i <= N; a[i] = i, i++);
	for(int i = 1; i <= M; i++){
		scanf("%d %d %lld", &pe[i].x, &pe[i].y, &pe[i].z);
	}
	kruskal();	
	init(1, 0, 0);
	for(int i = 1; i <= M; i++){
		if(!used[i]){
			int tx = pe[i].x, ty = pe[i].y, tz = pe[i].z;
			int add = LCA(tx, ty, tz);
			ans = min(ans, sum - add + tz);
		}
	}
	cout << ans;
	fclose(stdin);
	return 0;
}

loj上的结果：