Triangles

#include <fstream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
using namespace std;
typedef long long ll;
vector<ll> l;
ll k, k2;
ll s(ll c)
{
    return k + k2 * c*c - c*c*c*c;
}
bool isLeft(int i)
{
    return i + 1 < l.size() ? (s(l[i]) <= s(l[i + 1])) : (s(l[i]) >= s(l[i - 1]));
}
int main()
{
    ifstream in("input.txt");
    int n; in >> n;
    l.resize(n);
    for(int i = 0; i < n; i++) in >> l[i];
    in.close();
    sort(l.begin(), l.end());
    long long max_s = -1000000000;
    for(int i = 0; i < n - 2; i++)
        for(int j = i + 1; j < n - 1; j++)
        {
            ll a = l[i];
            ll b = l[j];
            k = -b*b*b*b + 2*a*a*b*b - a*a*a*a;
            k2 = 2*a*a + 2*b*b;
            int l_r = j + 1, r_r = n - 1;
            while(l_r != r_r)
            {
                if(r_r - 1 == l_r)
                    if(s(l[l_r]) > s(l[r_r]))
                        r_r = l_r;
                    else l_r = r_r;
                else
                {
                    int m = (l_r + r_r) / 2;
                    if(isLeft(m)) l_r = m;
                    else r_r = m;
                }
            }
            max_s = max(max_s, s(l[l_r]));
        }
    ofstream out("output.txt");
    if(max_s < 0) out << "Triangle not found\n";
    else out << fixed << setprecision(3) << sqrt(((long double)max_s) / 16.0) << "\n";
    out.close();
    return 0;
}