feat(codeforces/970): a-f

codeforces/970/f.cc

@@ -0,0 +1,58 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+using i64 = long long;
+constexpr i64 MOD = 1e9 + 7;
+
+i64 modinv(i64 a, i64 m) {
+  i64 m0 = m, y = 0, x = 1;
+  if (m == 1)
+    return 0;
+  while (a > 1) {
+    i64 q = a / m;
+    i64 t = m;
+    m = a % m, a = t;
+    t = y;
+    y = x - q * y;
+    x = t;
+  }
+  if (x < 0)
+    x += m0;
+  return x;
+}
+
+void solve() {
+  int n;
+  cin >> n;
+  vector<i64> a(n);
+  for (auto &x : a)
+    cin >> x;
+
+  i64 total_sum = 0, sum_sq = 0;
+  for (auto x : a) {
+    total_sum = (total_sum + x) % MOD;
+    sum_sq = (sum_sq + x * x) % MOD;
+  }
+
+  i64 sum_pairs = (total_sum * total_sum % MOD - sum_sq + MOD) % MOD;
+  sum_pairs = sum_pairs * modinv(2, MOD) % MOD;
+
+  i64 total_pairs = (i64)n * (n - 1) / 2 % MOD;
+  i64 inv_total_pairs = modinv(total_pairs, MOD);
+  i64 ans = sum_pairs * inv_total_pairs % MOD;
+
+  cout << ans << '\n';
+}
+
+int main() {
+  ios::sync_with_stdio(false);
+  cin.tie(nullptr);
+
+  int t;
+  cin >> t;
+  while (t--) {
+    solve();
+  }
+
+  return 0;
+}