782A - Andryusha and Socks - CodeForces Solution

#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
int main()
{

    ll n;cin>>n;
    ll m=2*n,counter=0,maxc=0;
    ll arr[m],brr[m]={0};for(ll i=0;i<m;i++)cin>>arr[i];
    for(ll i=0; i<m; i++){
        if(brr[arr[i]]==0){
            brr[arr[i]]++;
            counter++;
            if(counter>maxc)
                maxc=counter;
        }
        else{
            brr[arr[i]]--;
            counter--;
        }
    }
    cout << maxc << endl;
    return 0;
}