为什么我的代码用 c++20 提交超时，c++14 AC

代码：

#include<bits/stdc++.h>
using namespace std;
int a[3][3];
string End;
string to_str(int tmp[3][3]) {
	string str;
	for (int i=0;i<3;i++)
		for (int j=0;j<3;j++) str += tmp[i][j]+'0';
	return str; 
}
void output(unordered_map<string,string> ans) {
	string begin=to_str(a);
	stack<string> out;
	for (string str=End;;str=ans[str]) {
		out.push(str);
		if (str == begin) break;
	}
	while (!out.empty()) {
		string tmp=out.top(); out.pop();
		cout << tmp[0] << " " << tmp[1] << " " << tmp[2] << '\n';
		cout << tmp[3] << " " << tmp[4] << " " << tmp[5] << '\n';
		cout << tmp[6] << " " << tmp[7] << " " << tmp[8] << "\n\n"; 
	}
}
string opt1(string str) {
	char tmp=str[0];
	str[0]=str[3]; str[3]=str[6]; str[6]=str[7];
	str[7]=str[8]; str[8]=str[5]; str[5]=str[2];
	str[2]=str[1]; str[1]=tmp;
	return str;
}
string opt2(string str) {
	swap(str[3],str[5]);
	swap(str[4],str[5]);
	return str;
}
void bfs() {
	queue<pair<string,int> > q;
	unordered_map<string,string> ans;
	unordered_map<string,bool> maps;
	q.push({to_str(a),0}); maps[to_str(a)]=1;
	while (!q.empty()) {
		pair<string,int> tmp=q.front(); q.pop();
		if (tmp.first == End) {
			cout << tmp.second << '\n';
			output(ans);
			exit(0);
		}
		string t1=opt1(tmp.first);
		string t2=opt2(tmp.first);
		if (!maps[t1]) {
			maps[t1]=1; ans[t1]=tmp.first;
			q.push({t1,tmp.second+1}); 
		}
		if (!maps[t2]) {
			maps[t2]=1; ans[t2]=tmp.first;
			q.push({t2,tmp.second+1});
		}
	}
}
int main()
{
	for (int i=0;i<=8;i++) End += i+'0';
	for (int i=0;i<3;i++)
		for (int j=0;j<3;j++) cin >> a[i][j];
	bfs();
	cout << "UNSOLVABLE";
	return 0;
}