这个很明显暴力的算法居然测试点 $6 \sim 10$ 全能过

#include <bits/stdc++.h>
using namespace std;
int n,m,pos=0;
char c,x;
string arr[100005];
int main() {
	scanf("%d",&n);
	for (int i=1;i<=n;i++) {
		cin >> c;
		if (c=='T') {
			cin >> x;
			pos++;
			arr[pos]=arr[pos-1]+x;
		}
		else if (c=='Q') {
			scanf("%d",&m);
			cout << arr[pos][m-1] << endl;
		}
		else if (c=='U') {
			scanf("%d",&m);
			pos++;
			arr[pos]=arr[pos-m-1];
		}
	}
}