From 0895f652d6e7034e789f3559c35b121ebbe1df5c Mon Sep 17 00:00:00 2001
From: Ren Baoshuo <i@baoshuo.ren>
Date: Mon, 9 Aug 2021 21:14:30 +0800
Subject: [PATCH] =?UTF-8?q?P3799=20=E5=A6=96=E6=A2=A6=E6=8B=BC=E6=9C=A8?=
 =?UTF-8?q?=E6=A3=92?=
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R55412792
---
 Luogu/problem/P3799/P3799.cpp | 37 +++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)
 create mode 100644 Luogu/problem/P3799/P3799.cpp

diff --git a/Luogu/problem/P3799/P3799.cpp b/Luogu/problem/P3799/P3799.cpp
new file mode 100644
index 00000000..f7aac53c
--- /dev/null
+++ b/Luogu/problem/P3799/P3799.cpp
@@ -0,0 +1,37 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+const int mod = 1e9 + 7;
+
+int n, maxa, a[100005], num[100005];
+long long ans;
+
+long long C(long long x, long long k) {
+    return (k == 1ll ? x : x * (x - 1ll) / 2ll) % mod;
+}
+
+int main() {
+    cin >> n;
+    for (int i = 0; i < n; i++) {
+        cin >> a[i];
+        maxa = max(maxa, a[i]);
+        num[a[i]]++;
+    }
+    for (int i = 0; i < n; i++) {
+        if (num[i] >= 2ll) {
+            long long times = C(num[i], 2ll) % mod;
+            for (int j = 1; j <= i / 2; ++j) {
+                if (j != i - j && num[j] >= 1 && num[i - j] >= 1) {
+                    ans += times * C(num[j], 1) * C(num[i - j], 1) % mod;
+                }
+                if (j == i - j && num[j] >= 2) {
+                    ans += times * C(num[j], 2) % mod;
+                }
+                ans %= mod;
+            }
+        }
+    }
+    cout << ans << endl;
+    return 0;
+}