string echo()
{
    string ans = "";
    char a;
    while (cin >> a)//这里这里这里
    {
        if (a == '[')
        {
            int num;
            cin >> num;
            string t = echo();
            for (int i = 1; i <= num; i++)
                ans += t;
        }
        else if (a == ']')
            return ans;
        else
            ans += a;
    }
    return ans;
}
int main()
{
    cout << echo() << endl;//还有这里这里这里
    return 0;
}

为啥把这两个地方换成scanf("%c",&a)和printf("%s",echo())就会TLE qwq