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- int N;
- cin>>N;
- stack<string>stk;
- string st;
- while(N--){
- int n; cin>>n;
- if(n==1){
- string str;
- cin>>str;
- stk.push(st);
- st+=str;
- }
- else if(n==2){
- int k;cin>>k;
- stk.push(st);
- st.erase(st.size()-k);
- }
- else if(n==3){
- int k;cin>>k;
- cout<<st[k-1]<<endl;
- }
- else{
- st = stk.top();
- stk.pop();
- }
- }
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