1 files changed, 40 insertions, 0 deletions
diff --git a/2020/January/Gold/threesum.cpp b/2020/January/Gold/threesum.cpp
new file mode 100644
index 0000000..21e266c
--- /dev/null
+++ b/2020/January/Gold/threesum.cpp
@@ -0,0 +1,40 @@
+#include <bits/stdc++.h>
+#define f first
+#define s second
+using namespace std;
+typedef long long ll;
+typedef pair<int, int> ii;
+constexpr int MAX = 1e6;
+
+int A[5005], cnt[5005] = { 0 }, M[2000002];
+ll ans[100001] = { 0 };
+vector<pair<ii, int>> q;
+
+int main() {
+	ifstream cin("threesum.in");
+	ofstream cout("threesum.out");
+
+	int N, Q; cin >> N >> Q;
+	for (int i = 0; i < N; ++i) cin >> A[i];
+	for (int i = 0; i < Q; ++i) {
+		int a, b; cin >> a >> b;
+		if (a != b) q.emplace_back(ii(--a, --b), i);
+	}
+	sort(begin(q), end(q), [](const pair<ii, int>& a, const pair<ii, int>& b) {
+		return a.f.s < b.f.s || (a.f.s == b.f.s && a.f.f > b.f.f);
+	});
+	
+	auto it = begin(q);
+	for (int i = 0; i < N; ++i) {
+		ll sum = 0;
+		for (int j = i - 1; j >= 0; --j) {
+			int tmp = A[i] + A[j] + MAX;
+			if (tmp > 0 && tmp <= 2 * MAX && M[tmp]) cnt[j] += M[tmp];
+			++M[-A[j] + MAX];
+			sum += cnt[j];
+			while (it != end(q) && ii(j, i) == it->f) ans[(it++)->s] = sum;
+		}
+		for (int j = i - 1; j >= 0; --j) --M[-A[j] + MAX];
+	}
+	for (int i = 0; i < Q; ++i) cout << ans[i] << '\n';
+}