AtCoder C - Make a Rectangle

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

#define fast ios_base::sync_with_stdio(0)

typedef long long LL;
typedef vector<LL>vll;
typedef map<LL, LL>mplli;

vll v;
mplli mp;

set<LL>st;

int main()
{
    fast;
    int N;
    cin >> N;
    int i;
    for(i=0; i<N; i++)
    {
        LL x;
        cin >> x;
        st.insert(x);
        mp[x]++;
    }

    set<LL>::iterator it;

    for(it=st.begin(); it!=st.end(); it++)
    {
        // cout << *it << "; ";
        v.push_back(*it);
    }

    int len = v.size();

    LL mul = 1;
    int cnt = 0;

    if(mp[v[len-1]] >= 4) cout << v[len-1] * v[len-1] << "\n";
    else
    {
        cnt = 0;
        mul = 1;

        for(i=len-1; i>=0; i--)
        {
            if(mp[v[i]] >= 2)
            {
                cnt+=1;
                mul *= v[i];
            }

            if(cnt == 2) break;
        }

        if(cnt == 2) cout << mul << "\n";
        else cout << "0\n";
    }

    return 0;
}