// Problem: P1928 外星密码
// Contest: Luogu
// URL: https://www.luogu.com.cn/problem/P1928
// Memory Limit: 125 MB
// Time Limit: 1000 ms
// 
// Powered by CP Editor (https://cpeditor.org)

#include <bits/stdc++.h>
using namespace std;

string solve()
{
	int k;
	string s = "";
	string s0;
	char temp;
	while (cin >> temp)
	{
		if (temp == '[')
		{
			cin >> k;
			s0 = solve();
			for (int i = 0; i < k; i++)
			{
				s += s0;
			}
		}
		else
		{
			if (temp == ']')
			{
				return s;
			}
			s += temp;
		}
	}
}

int main()
{
	cout << solve();
	return 0;
}

开了O2，上面的代码直接全部RE，但是没开O2直接AC。如果开了O2应该怎么改？