标题效果：两棵树之间的首次查询k大点的权利。

思维：树木覆盖树，事实上，它是正常的树木覆盖了持久段树。

由于使用权值段树可以寻求区间k大，然后应用到持久段树思想，间隔可以做减法。详见代码。

CODE：

#include <cstdio>

#include <cstring>

#include <iostream>

#include <algorithm>

#define MAX 100010

#define NIL (tree[0])

using namespace std;

pair<int,int> src[MAX];

struct Complex{

	Complex *son[2];

	int cnt;

	Complex(Complex *_,Complex *__,int ___):cnt(___) {

		son[0] = _;

		son[1] = __;

	}

	Complex() {}

}*tree[MAX];

int points,asks;

int xx[MAX];

int head[MAX],total;

int next[MAX << 1],aim[MAX << 1];

int father[MAX][20],deep[MAX];

inline void Add(int x,int y);

void DFS(int x);

void SparseTable();

Complex *BuildTree(Complex *pos,int x,int y,int val);

int GetAns(Complex *l,Complex *r,Complex *f,Complex *p,int x,int y,int k);

int GetLCA(int x,int y);

int Ask(int x,int y,int k);

int main()

{

	cin >> points >> asks;

	for(int i = 1;i <= points; ++i)

		scanf("%d",&src[i].first),src[i].second = i;

	sort(src + 1,src + points + 1);

	for(int i = 1;i <= points; ++i)

		xx[src[i].second] = i;

	for(int x,y,i = 1;i < points; ++i) {

		scanf("%d%d",&x,&y);

		Add(x,y),Add(y,x);

	}

	NIL = new Complex(NULL,NULL,0);

	NIL->son[0] = NIL->son[1] = NIL;

	DFS(1);

	SparseTable();

	int now = 0;

	for(int x,y,k,i = 1;i <= asks; ++i) {

		scanf("%d%d%d",&x,&y,&k);

		printf("%d",now = src[Ask(x ^ now,y,k)].first);

		if(i != asks)	puts("");

	}

	return 0;

}

inline void Add(int x,int y)

{

	next[++total] = head[x];

	aim[total] = y;

	head[x] = total;

}

void DFS(int x)

{

	deep[x] = deep[father[x][0]] + 1;

	tree[x] = BuildTree(tree[father[x][0]],1,points,xx[x]);

	for(int i = head[x];i;i = next[i]) {

		if(aim[i] == father[x][0])	continue;

		father[aim[i]][0] = x;

		DFS(aim[i]);

	}

}

void SparseTable()

{

	for(int j = 1;j <= 19; ++j)

		for(int i = 1;i <= points; ++i)

			father[i][j] = father[father[i][j - 1]][j - 1];

}

int Ask(int x,int y,int k)

{

	int lca = GetLCA(x,y);

	return GetAns(tree[x],tree[y],tree[lca],tree[father[lca][0]],1,points,k);

}

Complex *BuildTree(Complex *pos,int x,int y,int val)

{

	int mid = (x + y) >> 1;

	if(x == y)	return new Complex(NIL,NIL,pos->cnt + 1);

	if(val <= mid)	return new Complex(BuildTree(pos->son[0],x,mid,val),pos->son[1],pos->cnt + 1);

	return new Complex(pos->son[0],BuildTree(pos->son[1],mid + 1,y,val),pos->cnt + 1);

}

int GetLCA(int x,int y)

{

	if(deep[x] < deep[y])	swap(x,y);

	for(int i = 19; ~i; --i)

		if(deep[father[x][i]] >= deep[y])

			x = father[x][i];

	if(x == y)	return x;

	for(int i = 19; ~i; --i)

		if(father[x][i] != father[y][i])

			x = father[x][i],y = father[y][i];

	return father[x][0];

}

int GetAns(Complex *l,Complex *r,Complex *f,Complex *p,int x,int y,int k)

{

	if(x == y)	return x;

	int mid = (x + y) >> 1;

	int temp = l->son[0]->cnt + r->son[0]->cnt - f->son[0]->cnt - p->son[0]->cnt;

	if(k <= temp)	return GetAns(l->son[0],r->son[0],f->son[0],p->son[0],x,mid,k);

	return GetAns(l->son[1],r->son[1],f->son[1],p->son[1],mid + 1,y,k - temp);

}