rt，平衡树的操作没有问题，不知道那里写挂了。

https://www.luogu.com.cn/record/97113161

#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 5;
struct node {
	int val, ls, rs, cnt, tot, siz, tsiz;
} tree[N];
int cnt, tot, cur[N];
queue<int> delq;
int new_node() {
    if (delq.empty()) return ++ cnt;
    int res = delq.front(); delq.pop();
    return res;
}
struct ScapegoatTree {
	const double alpha = 0.6666;
	int siz, rt;
    ScapegoatTree() { siz = rt = 0; }
	bool isBad(int u) {
		return tree[u].cnt && (alpha * tree[u].tot <= double(max(tree[tree[u].ls].tot, tree[tree[u].rs].tot))
			|| double(tree[u].tsiz) < alpha * tree[u].tot);
	}
	void push_up(int u) {
		tree[u].tot = tree[tree[u].ls].tot + tree[tree[u].rs].tot + 1;
		tree[u].siz = tree[tree[u].ls].siz + tree[tree[u].rs].siz + tree[u].cnt;
		tree[u].tsiz = tree[tree[u].ls].tsiz + tree[tree[u].rs].tsiz + bool(tree[u].cnt);
	}
	void dfs(int u) {
		if (!u) return;
		dfs(tree[u].ls);
		if (tree[u].cnt) cur[tot ++] = u;
		dfs(tree[u].rs);
	}
	int build(int l, int r) {
		int mid = (l + r) >> 1;
		if (l >= r) return 0;
		tree[cur[mid]].ls = build(l, mid);
		tree[cur[mid]].rs = build(mid + 1, r);
		push_up(cur[mid]);
		return cur[mid];
	}
	inline void rebuild(int& u) {
		tot = 0, dfs(u);
		if (u == rt) rt = u = build(0, tot);
		else u = build(0, tot);
	}
	int insert(int u, int val) {
		if (!u) {
            u = new_node();
			if (!rt) rt = u;
			tree[u].val = val;
			tree[u].siz = tree[u].cnt = 1;
		} else {
			if (tree[u].val == val) tree[u].cnt ++;
			if (val < tree[u].val) tree[u].ls = insert(tree[u].ls, val);
			if (val > tree[u].val) tree[u].rs = insert(tree[u].rs, val);
			push_up(u);
			if (isBad(u)) rebuild(u);
		}
		return u;
	}
	void del(int u, int val) {
		if (!u) return;
		if (tree[u].val == val) tree[u].cnt --;
		if (val < tree[u].val) del(tree[u].ls, val);
		if (val > tree[u].val) del(tree[u].rs, val);
		push_up(u);
	}
	int query(int u, int val) {
		if (!u) return 0;
		if (tree[u].val == val) return tree[tree[u].ls].siz;
		if (val < tree[u].val) return query(tree[u].ls, val);
		return query(tree[u].rs, val) + tree[tree[u].ls].siz + tree[u].cnt;
	}
	int Kth(int u, int k) {
		if (!u) return -1;
		if (tree[tree[u].ls].siz >= k) return Kth(tree[u].ls, k);
		if (tree[tree[u].ls].siz + tree[u].cnt >= k) return tree[u].val;
		return Kth(tree[u].rs, k - tree[tree[u].ls].siz - tree[u].cnt);
	}
    void merge(int u) {
        if (!u) return;
        for (int i = 1; i <= tree[u].cnt; i ++)
            insert(rt, tree[u].val), delq.push(u);
        merge(tree[u].ls), merge(tree[u].rs);
    }  
	void print(int u) {
		if (!u) return;
		print(tree[u].ls);
		print(tree[u].rs);
		cout << tree[u].val << ' ' << tree[u].cnt << '\n';
	}
} T[N];
int n, m, q, u, v, f[N], rk[N];
char opt;
int find(int u) {
    return f[u] == u ? u : f[u] = find(f[u]);
}
void merge(int u, int v) {
    int fu = find(u), fv = find(v);
    if (fu == fv) return;
    if (T[fu].siz < T[fv].siz) swap(fu, fv);
    T[fu].merge(T[fv].rt), f[fv] = fu;
}
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);
	cin >> n >> m;
    for (int i = 1; i <= n; i ++)
        cin >> u, rk[u] = i, T[i].insert(T[i].rt, u), f[i] = i;
    for (int i = 1; i <= m; i ++)
        cin >> u >> v, merge(u, v);
    cin >> q;
	for (int i = 1; i <= q; i ++) {
		cin >> opt >> u >> v;
        if (opt == 'Q') {
            u = find(u);
            int res = T[u].Kth(T[u].rt, v);
            cout << (~res ? rk[res] : res) << '\n';
        }
        if (opt == 'B') merge(u, v);
        if (opt == 'C') u = find(u), T[u].print(T[u].rt);
	}
	return 0;
}