H - Pair: normal and paranormal

 URAL - 2019 

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int n,cnt=1;
    string str;
    stack<pair<char,int> >stk;
    map<int,int>mmp;
    map<int,int>kill;
    cin>>n>>str;
    for(int i=0; i<2*n; i++)
    {
        if(str[i]>='a'&&str[i]<='z')
            mmp[i]=cnt++;
    }
    for(int i=0; i<2*n; i++)
    {
        if(!stk.empty())
        {
            if(fabs(str[i]-stk.top().first)==32)
            {
                if((str[i]>='a'&&str[i]<='z')&&(stk.top().first>='A'&&stk.top().first<='Z'))
                {
                    kill[stk.top().second]=i;
                    stk.pop();
                }
                else if((str[i]>='A'&&str[i]<='Z')&&(stk.top().first>='a'&&stk.top().first<='z'))
                {
                    kill[i]=stk.top().second;
                    stk.pop();
                }
                else
                    stk.push(make_pair(str[i],i));
            }
            else
                stk.push(make_pair(str[i],i));
        }
        else
            stk.push(make_pair(str[i],i));
    }
    cnt=0;
    if(!stk.empty())
        cout<<"Impossible"<<endl;
    else
    {
        for(int i=0; i<2*n; i++)
        {
            if(str[i]>='A'&&str[i]<='Z')
            {
                if(cnt>0)
                    cout<<" "<<mmp[kill[i]];
                else
                    cout<<mmp[kill[i]];
                cnt++;
            }
        }
        cout<<endl;
    }
    return 0;
}