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  1. #include<bits/stdc++.h>
  2. using namespace std;
  3.  
  4. #define ll long long
  5.  
  6. void solve(){
  7.     int n;
  8.     cin>>n;
  9.     vector<int>v(n);
  10.     for(int i=0;i<n;i++)cin>>v[i];
  11.     // v[i]>v[j]<v[k]>v[l] i < j < k < l
  12.     vector<int>pref(n,0),suf(n,0);
  13.     for(int i=0;i<n;i++){
  14.         for(int j=0;j<i;j++){
  15.             if(v[j]>v[i])pref[i]++;
  16.         }
  17.     }
  18.     for(int i=n-1;i>=0;i--){
  19.         for(int j=i+1;j<n;j++){
  20.             if(v[j]<v[i])suf[i]++;
  21.         }
  22.     }
  23.     int ans=0;
  24.     for(int i=1;i<n-2;i++){
  25.         for(int j=i+1;j<n-1;j++){
  26.             if(v[i]<v[j]){
  27.                 ans+=pref[i]*suf[j];
  28.             }
  29.         }
  30.     }
  31.     cout<<ans<<"\n";
  32. }
  33.  
  34. int main(){
  35.     ios_base::sync_with_stdio(false);
  36.     cin.tie(NULL);
  37.     solve();
  38.     return 0;
  39. }
Success #stdin #stdout 0.01s 5324KB
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https://ideone.com/1n4OaU
language:
C++14 (gcc 8.3)
created:
15 days ago
visibility:
public

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