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  1. // 1,4,-1,2  targ = 4 
  2. // index 1,4 |v[1]-v[4]|+|v[1]+v[4]|=target
  3. //            [1-2] + [1+2]
  4. //            |-1| + |3|
  5. //             4
  6. // index 3,4 |v[3]-v[4]|+|v[3]+v[4]|
  7. //           |-1-2|+|-1+2|
  8. //           |-3| + |1|
  9. //            4
  10. #include<bits/stdc++.h>
  11. using namespace std;
  12.  
  13. #define ll long long
  14.  
  15. void solve(){
  16.     ll n,target;
  17.     cin>>n;
  18.     vector<ll>v(n);
  19.     for(ll &i:v)cin>>i;
  20.     cin>>target;
  21.  
  22.     if(target&1){
  23.         cout<<0<<"\n";
  24.         return;
  25.     }
  26.     ll half_target=target/2;
  27.  
  28.     unordered_map<ll,ll>mp;
  29.     for(ll i:v)mp[abs(i)]++;
  30.  
  31.     ll half_target_element=mp[half_target],less_than_half_target=0;
  32.     for(auto &i:mp){
  33.         if(i.first<half_target)less_than_half_target+=i.second;
  34.     }
  35.  
  36.     ll ans=less_than_half_target*half_target_element;
  37.     ans+=(half_target_element*(half_target_element-1))/2;
  38.     cout<<ans<<"\n";
  39. }
  40.  
  41. int main(){
  42.     ios_base::sync_with_stdio(false);
  43.     cin.tie(NULL);
  44.     solve();
  45.     return 0;
  46. }
Success #stdin #stdout 0s 5320KB
comments (?)
stdin
copy 
4
1 -2 3 2
4
stdout
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3
https://ideone.com/FjdgDs
language:
C++14 (gcc 8.3)
created:
1 day ago
visibility:
public

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