避坑！先定义栈即可ac

include<stack>
#include<string>
#include<iostream>
using namespace std;
string s,b;//先定义了字符串，会RE
stack<int>q;
int main()
{
    cin>>s;
    int l=s.size();
    for(int i=0;i<l;i++)
    {
        if(s[i]=='(')
        {
            q.push(i);
            b[i]=')';
        }
        if(s[i]=='[')
        {
            q.push(i);
            b[i]=']';
        }
        if(s[i]==')' or s[i]==']')
            if(q.empty() or b[q.top()]!=s[i])           
                if(s[i]==')') 
                    b[i]='(';
                else 
                    b[i]='[';           
            else b[q.top()]=' ',q.pop();                                                            
    }
    for(int i=0;i<l;i++)
    {
        if(b[i]=='('||b[i]=='[')
            cout<<b[i];
        cout<<s[i];
        if(b[i]==')'||b[i]==']')
            cout<<b[i];
    }
    cout<<endl;
}