奇怪啊，怎么过不去呢，每次运行并查集的 merge 操作 就会寄，大概是treap启发式合并写挂了？我没看出来哪儿挂了

#include<bits/stdc++.h>
#include<chrono>
using namespace std;
mt19937 mt(chrono::system_clock::to_time_t(chrono::system_clock::now()));

const long long N=5e5+10;
int n,m,q;

struct treap{
	int lc[N],rc[N],val[N],w[N],siz[N],id[N];
	int root[N],cnt;
	int newnode(int v,int i){
		cnt++;
		val[cnt]=v;
		w[cnt]=(int)mt();
		siz[cnt]=1;
		id[cnt]=i;
	}
	void pushup(int x){
		siz[x]=siz[lc[x]]+siz[rc[x]]+1;
	}
	void split(int x,int v,int &a,int &b){
		if(!a||!b){
			a=b=0;
			return;	
		}
		if(v>=val[x]){
			b=x;
			split(lc[x],v,a,lc[0]);
		}else{
			a=x;
			split(rc[x],v,rc[x],b);
		}
		pushup(x);
	}
	int merge(int x,int y){
		if(!x||!y) return x+y;
		if(w[x]<w[y]){
			rc[x]=merge(rc[x],y);
			pushup(x);
			return x;
		}else{
			lc[y]=merge(x,lc[y]);
			pushup(y);
			return y;
		}
	}
	void split2(int x,int k,int &a,int &b){
		if(!x){
			a=b=0;
			return;
		}
		if(k<=siz[lc[x]]){
			b=x;
			split2(lc[x],k,a,lc[x]);
		}else{
			a=x;
			split2(rc[x],k-siz[lc[x]]-1,rc[x],b);
		}
		pushup(x);
	}
}fhq;


struct UFS{
	int fa[N];
	void init(){
		for(int i=1;i<=n;i++) fa[i]=i;
	}
	int getfa(int x){
		if(fa[x]==x) return x;
		return fa[x]=getfa(fa[x]);
	}
	bool check(int &x,int &y){
		x=getfa(x);
		y=getfa(y);
		return x==y;
	}
	void dfs(int x,int &y){
		if(!x) return;
		dfs(fhq.lc[x],y);
		dfs(fhq.rc[x],y);
		fhq.lc[x]=fhq.rc[x]=0;
		fhq.siz[x]=1;
		int t=fhq.val[x];
		int rt1,rt2;
		fhq.split(y,t,rt1,rt2);
		y=fhq.merge(rt1,fhq.merge(x,rt2));
	}
	void merge(int x,int y){
		if(check(x,y)) return;
		if(fhq.siz[fhq.root[x]]>fhq.siz[fhq.root[y]]) swap(x,y);//这句话莫名其妙会寄
		fa[x]=y;
		dfs(fhq.root[x],fhq.root[y]);//这里也会寄
	}
}ufs;

int main(){
	cin>>n>>m;
	ufs.init();
	for(int i=1;i<=n;i++){
		int x;
		cin>>x;
		fhq.root[i]=fhq.newnode(x,i);
	}
	for(int i=1;i<=m;i++){
		int u,v;
		cin>>u>>v;
		ufs.merge(u,v);//就是这儿挂了
	}
	cin>>q;
	for(int i=1;i<=q;i++){
		char op;
		int u,v;
		cin>>op>>u>>v;
		if(op=='Q'){
			u=ufs.getfa(u);
			if(fhq.siz[fhq.root[u]]<v){
				cout<<"-1\n"; continue;
			}
			int t1,t2,t3;
			fhq.split2(fhq.root[u],fhq.siz[fhq.root[u]]-v,t1,t2);
			fhq.split2(t2,1,t2,t3);
			cout<<fhq.id[t2]<<endl;
			fhq.root[u]=fhq.merge(t1,fhq.merge(t2,t3));
		}else{
			ufs.merge(u,v);
		}
	}
}