my c++ sol:

#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
#include <stack>
using namespace std;


int main() {
    /* Enter your code here. Read input from STDIN. Print output to STDOUT */
    string s="";
    stack<string> st;
    int n;
    cin>>n;
    for(int i =0; i< n; i++)
    {
        int k =0;
        cin>>k;
        if(k==1)
        {
            string str;
            cin>>str;
            st.push(s);
            s.append(str);
        }
        if(k==2)//last k character
        {
            int no;
            cin>>no;
            st.push(s);
            while(no>0)
            {
                s.pop_back();
                no--;
            }
        }
        if(k==3)//print kth char
        {
            int h;
            cin>>h;
            cout<<s[h-1]<<endl;
        }
        else if (k == 4) {
            if (!st.empty()) {
                s = st.top();
                st.pop();
            }
        }
    }
    return 0;
}