D - Triangles atcoder

#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pb push_back
#define SZ(v) ((int)((v).size()))
constexpr ll mod = 1e9+7;
constexpr ll inf = 1e18+7;
void run () {
    int n;
    cin >> n;
    vector<ll> L(n);
    for (ll &i : L)
        cin >> i;
    sort(L.begin(), L.end());
    ll ans = 0;
    for (int i = 0; i < n; ++i) {
        for (int j = i + 1; j < n; ++j) {
            ll sumOfpair = L[i] + L[j];
            int k = lower_bound(L.begin(), L.end(), sumOfpair) - L.begin();
            --k;
            ans += max(0, k - j);
        }
    }
    cout << ans << '\n';
}
int main () {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
//    freopen("/home/hamoudi/clion/hello.in", "rt", stdin);
    int tt;
    tt = 1;
//    cin >> tt;
    while (tt--)
        run();
}
/*

 */