好久没写过这样的线性dp了，其实方程很简单。。。就是太懒了没画草稿纸，才写错方程了吧。。。wa了好几发，惩罚自己写个分享

char s[N];
int f[N];
void solve()
{
    cin >> (s + 1);    
    int n = strlen(s + 1);
    for (int i = 1; i <= n; i++)
    {
        if (s[i] == '(' || s[i] == '[') continue;
        else 
        {
            if ( (s[i - f[i - 1] - 1] == '[' && s[i] == ']') 
            ||   (s[i - f[i - 1] - 1] == '(' && s[i] == ')') )
            {
                f[i] = f[i - 1] + f[i - f[i - 1] - 1 - 1] + 2;
            }
        }
    }
    int ans = *max_element(f + 1, f + 1 + n);
    for (int i = 1; i <= n; i++) 
    {
        if (ans == f[i])
        {
            for (int j = i - ans + 1; j <= i; j++) cout << s[j];
            return;
        }
    }
}