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P3799 妖梦拼木棒
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Luogu/problem/P3799/P3799.cpp
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37
Luogu/problem/P3799/P3799.cpp
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#include <bits/stdc++.h>
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using namespace std;
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const int mod = 1e9 + 7;
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int n, maxa, a[100005], num[100005];
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long long ans;
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long long C(long long x, long long k) {
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return (k == 1ll ? x : x * (x - 1ll) / 2ll) % mod;
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}
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int main() {
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cin >> n;
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for (int i = 0; i < n; i++) {
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cin >> a[i];
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maxa = max(maxa, a[i]);
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num[a[i]]++;
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}
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for (int i = 0; i < n; i++) {
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if (num[i] >= 2ll) {
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long long times = C(num[i], 2ll) % mod;
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for (int j = 1; j <= i / 2; ++j) {
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if (j != i - j && num[j] >= 1 && num[i - j] >= 1) {
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ans += times * C(num[j], 1) * C(num[i - j], 1) % mod;
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}
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if (j == i - j && num[j] >= 2) {
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ans += times * C(num[j], 2) % mod;
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}
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ans %= mod;
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}
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}
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}
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cout << ans << endl;
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return 0;
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}
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