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OI-codes/Luogu/P3799/P3799.cpp

38 lines
909 B
C++

#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;
}