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  1. #include <bits/stdc++.h>
  2. using namespace std;
  3. using ll = long long;
  4.  
  5. int main() {
  6.     ios::sync_with_stdio(false);
  7.     cin.tie(nullptr);
  8.     freopen("SUM.INP" , "r" , stdin);
  9.     freopen("SUM.OUT" , "w" , stdout);
  10.     ll n;
  11.     cin >> n;
  12.     vector<ll> a(n);
  13.     for(int i = 0; i < n; i++) cin >> a[i];
  14.     unordered_map<ll, ll> freq;
  15.     ll dem = 0;
  16.     for(int i = 0; i < n; i++) {
  17.         dem += freq[-a[i]];
  18.         if(a[i] == 0) dem++; 
  19.  
  20.         freq[a[i]]++;
  21.     }
  22.  
  23.     cout << dem;
  24.     return 0;
  25. }
  26.  
Success #stdin #stdout 0s 5324KB
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stdin
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5
-3 1 0 -1 3
stdout
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3
https://ideone.com/P9Q9vk
language:
C++ (gcc 8.3)
created:
4 days ago
visibility:
public

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