rt，样例对了，substak1过了，substak0只对了第一个点

#include<bits/stdc++.h>
#define f(i,a,b) for(int i = a ; i<=b ; i++) 
using namespace std;
const int N = 5e4+5;
struct node{
	int u,v,w;
}e[N];
int n,m,q,x,y,z;
int rt[N],dep[N],fa[N][20],a[N][20];
bool vis[N];
bool cmp(node a,node b){return a.w>b.w;}
vector<node> G[N];
int find_rt(int x){
	if(x == rt[x]) return x;
	return rt[x] = find_rt(rt[x]);
}
void kruskal(){
	f(i,1,n) rt[i] = i;
	int cnt = 0;
	f(i,1,m){
		int u = e[i].u,v = e[i].v,w = e[i].w;
		int fx = find_rt(u),fy = find_rt(v);
		if(fx == fy) continue;
		cnt++;
		rt[fx] = fy;
		G[u].push_back(node{0,v,w});
		G[v].push_back(node{0,u,w});
		if(cnt == n-1) break;
	}
}
void dfs(int u,int f){
	fa[u][0] = f;
	dep[u] = dep[f]+1;
	vis[u] = 1;
	for(auto e:G[u]){
		int v = e.v,w = e.w;
		if(v == f) continue;
		a[v][0] = w;
		dfs(v,u);
	} 
}
int lowbit(int x){
	return x&(-x);
}
int LCA(int x,int y){
	int ans = 2e9;
	if(find_rt(x) != find_rt(y)) return -1;
	if(dep[x]<dep[y]) swap(x,y);
	for(int i = dep[x]-dep[y] ; i ; i-=lowbit(i)){
		int k = log2(lowbit(i));
		ans = min(ans,a[x][k]);
		x = fa[x][k];
	}
	if(x == y) return ans;
	for(int i = log2(n) ; i>=0 ; i--){
		if(fa[x][i] == fa[y][i]) continue;
		ans = min(ans,a[x][i]);
		ans = min(ans,a[y][i]);
		x = fa[x][i];
		y = fa[y][i];
	}
	ans = min(ans,min(a[x][0],a[y][0]));
	return ans;
}
int main(){
	cin>>n>>m;
	f(i,1,n) rt[i] = i;
	f(i,1,m){
		cin>>x>>y>>z;
		e[i] = node{x,y,z};
	}
	sort(e+1,e+m+1,cmp);
	kruskal();
	f(i,1,n) if(!vis[i]) dfs(i,i); 
	f(j,1,log2(n)){
		f(i,1,n){
			fa[i][j] = fa[fa[i][j-1]][j-1];
			a[i][j] = min(a[a[i][j-1]][j-1],a[i][j-1]);
		}
	}
	cin>>q;
	while(q--){
		cin>>x>>y;
		cout<<LCA(x,y)<<endl;
	}
 	return 0;
}