I did my best to solve this problem. I tried various methods, but I accepted the same problem on another Luogu, but this problem had a runtime error
I come from China and my English is not good. I have done my best to translate 
#include<bits/stdc++.h>
#define int long long
using namespace std;
//
vector<int>e[222222];
int head[222222];
void add_edge(int x,int y)
{
e[x].push_back(y);
e[y].push_back(x);
}
//
int cnt;
struct node1{
int l,r,v;
}tr[222222];
void build(node1 &root,int l,int r)
{
root.v=0;
if(l==r)
{
return;
}
int mid=(l+r)/2;
build(tr[root.l=++cnt],l,mid);
build(tr[root.r=++cnt],mid+1,r);
}
int rts;
int a[222222];
void change(node1 rt1,node1 &rt2,int l,int rr,int x)
{
rt2.v=rt1.v+1;
if(l==rr)
{
return;
}
int mid=(l+rr)/2;
if(x<=mid)
{
change(tr[rt1.l],tr[rt2.l=++cnt],l,mid,x);
rt2.r=rt1.r;
}
else
{
change(tr[rt1.r],tr[rt2.r=++cnt],mid+1,rr,x);
rt2.l=rt1.l;
}
}
int sum(node1 rt1,node1 rt2,node1 rt3,node1 rt4,int l,int rr,int x)
{
if(l==rr)
{
return l;
}
int ss=tr[rt1.l].v+tr[rt2.l].v-tr[rt3.l].v-tr[rt4.l].v,mid=(l+rr)/2;
if(x<=ss)
{
return sum(tr[rt1.l],tr[rt2.l],tr[rt3.l],tr[rt4.l],l,mid,x);
}
else
{
return sum(tr[rt1.r],tr[rt2.r],tr[rt3.r],tr[rt4.r],mid+1,rr,x-ss);
}
}
int dep[111111];
int st[111111][22];
int lca(int x,int y)
{
if(dep[x]>dep[y])
{
swap(x,y);
}
for(int i=18;i>=0;i--)
{
if(dep[st[y][i]]>=dep[x])
{
y=st[y][i];
}
}
if(x==y)
{
return x;
}
for(int i=18;i>=0;i--)
{
if(st[y][i]!=st[x][i])
{
x=st[x][i];
y=st[y][i];
}
}
return st[y][0];
}
int n,m;
int aa[111111],b[111111];
int b_len;
void dfs(int x,int fa)
{
change(tr[head[fa]],tr[head[x]=++cnt],1,b_len,(lower_bound(b+1,b+1+b_len,aa[x])-b));
st[x][0]=fa;
dep[x]=dep[fa]+1;
for(int i=1;i<=18;i++)
{
st[x][i]=st[st[x][i-1]][i-1];
}
// for(auto y:e[x])
for(vector<int>::iterator i=e[x].begin();i!=e[x].end();i++)
{
int y=*i;
if(y==fa)
{
continue;
}
dfs(y,x);
}
}
int last=0;
signed main()
{
// cin>>n>>m;
scanf("%lld%lld",&n,&m);
for(int i=1;i<=n;i++)
{
// cin>>aa[i];
scanf("%lld",&aa[i]);
b[i]=aa[i];
}
sort(b+1,b+1+n);
b_len=unique(b+1,b+n+1)-(b+1);
build(tr[head[0]=++cnt],1,b_len);
for(int i=1,x,y;i<n;i++)
{
// cin>>x>>y;
scanf("%lld%lld",&x,&y);
add_edge(x,y);
add_edge(y,x);
}
dfs(1,0);
for(int i=1,x,y,z;i<=m;i++)
{
// cin>>x>>y>>z;
scanf("%lld%lld%lld",&x,&y,&z);
int la=lca(x,y);
// cout<<b[sum(tr[head[x]],tr[head[y]],tr[head[la]],tr[head[st[la][0]]],1,b_len,z)]<<'\n';
printf("%lld\n",b[sum(tr[head[x]],tr[head[y]],tr[head[la]],tr[head[st[la][0]]],1,b_len,z)]);
}
}
