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  1. #include<bits/stdc++.h>
  2. #define ll long long
  3. #define task ""
  4. using namespace std;
  5. const ll N=1e6+11;
  6. const ll mod=1e9+7;
  7. ll n,a[N],d[4*N],ans=0;
  8. void solve(){
  9.     cin>>n;
  10.     for(ll i=1;i<=n;i++){
  11.         cin>>a[i];
  12.     }
  13.     for(ll i=1;i<=n;i++){
  14.         for(ll j=i+1;j<=n;j++){
  15.             ans=(ans+d[-a[i]+a[j]+N])%mod;
  16.             ans%=mod;
  17.         }
  18.         for(ll j=1;j<=i-1;j++){
  19.             d[a[i]+a[j]+N]++;
  20.         }
  21.     }
  22.     cout<<ans;
  23. }
  24. int main(){
  25.     ios_base::sync_with_stdio(false);
  26.     cin.tie(0);cout.tie(0);
  27.     freopen("TSUM.INP","r",stdin);
  28.     freopen("TSUM.OUT","w",stdout);
  29.     solve();
  30. }
  31.  
Success #stdin #stdout 0.01s 5284KB
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https://ideone.com/SPjkNu
language:
C++14 (gcc 8.3)
created:
7 days ago
visibility:
public

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