Balance the Parenthesis

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

int main()
{
    string s;
    cin>>s;

    int n = s.length();

    // stack to store indices where the opening brackets occured
    stack<int> st;
    st.push(0); // '0' pushed for reference to handle base cases

    // to store positions where opening brackets to be pushed
    vector<int> store;

    for(int i=0;i<n;i++)
    {
        if(s[i]=='{'){
            st.push(i); // push the position of opening bracket
        }
        else if(st.size()>1){
            st.pop(); // we have a balanced bracket here
        }
        else{
            // we have reached a state where we have closing bracket but we don't have
            // any opening bracket that has occured earlier
            // push position for opening bracket
            store.push_back(st.top());
        }
    }
    int j=0;
    string ans = "";

    for(int i=0;i<n;i++)
    {
        // push oepning bracket till we have the condition being true
        while(j<store.size() and store[j]==i){
            ans+='{';
            j++;
        }
        ans+=s[i];
    }

    // consider the case where stack size is greater than 1
    // it means we have some positions of opening brackets but we don't have positions of closing brackets
    // we need to add that
    while(st.size()>1){
        ans+='}';
        st.pop();
    }
    cout<<ans;
}