1 files changed, 45 insertions, 0 deletions
diff --git a/20/day2/triangles.cpp b/20/day2/triangles.cpp
new file mode 100644
index 0000000..c30cbc7
--- /dev/null
+++ b/20/day2/triangles.cpp
@@ -0,0 +1,45 @@
+#include <bits/stdc++.h>
+#define f first
+#define s second
+using namespace std;
+typedef long long ll;
+typedef pair<int, int> ii;
+constexpr ll MOD = 1e9+7;
+
+ii P[2002];
+
+int main() {
+	ifstream cin("triangles.in");
+	ofstream cout("triangles.out");
+
+	int N;
+	cin >> N;
+	for (int i = 0; i < N; ++i) cin >> P[i].f >> P[i].s;
+	sort(P, P + N);
+
+	ll ans = 0;
+	for (int i = 0; i < N; ++i) {
+		ii p = P[i];
+		vector<ii> l, r;
+		for (int j = 0; j < i; ++j) l.push_back(P[j]);
+		for (int j = i + 1; j < N; ++j) r.push_back(P[j]);
+		sort(l.begin(), l.end(), [&p](const ii& a, const ii& b) {
+			int x = (a.f - p.f) * (b.s - p.s) - (a.s - p.s) * (b.f - p.f);
+			if (x == 0) return (a.f - p.f) * (a.f - p.f) + (a.s - p.s) * (a.s - p.s) < (b.f - p.f) * (b.f - p.f) + (b.s - p.s) * (b.s - p.s);
+			return x < 0;
+		});
+		sort(r.begin(), r.end(), [&p](const ii& a, const ii& b) {
+			int x = (a.f - p.f) * (b.s - p.s) - (a.s - p.s) * (b.f - p.f);
+			if (x == 0) return (a.f - p.f) * (a.f - p.f) + (a.s - p.s) * (a.s - p.s) < (b.f - p.f) * (b.f - p.f) + (b.s - p.s) * (b.s - p.s);
+			return x < 0;
+		});
+
+		for (int j = 0, k = 0; j < i; ++j) {
+			while (k < N - i - 1 && (l[j].f - p.f) * (r[k].s - p.s) - (l[j].s - p.s) * (r[k].f - p.f) <= 0) ++k;
+			ans += (ll)(2 * j + N - 2 * i - 2 * k) * (p.s + l[j].s) * (p.f - l[j].f) + MOD * MOD;
+			ans %= MOD;
+		}
+	}
+
+	cout << ans << '\n';
+}
+\ No newline at end of file