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  1. #include <bits/stdc++.h>
  2. #define N 100005
  3. using namespace std;
  4.  
  5. long long n, a[N];
  6.  
  7. void sub1()
  8. {
  9.     long long cnt = 0;
  10.     for(int i = 1; i <= n; i++)
  11.     {
  12.         if(a[i] + a[i+1] > a[n]) break;
  13.         for(int j = i + 1; j <= n; j++)
  14.         {
  15.             if(a[i] + a[j] > a[n]) break;
  16.             if(binary_search(a + j + 1, a + 1 + n, a[i] + a[j]))
  17.             {
  18.                 //cout << a[i] << ' ' << a[j] << '\n';
  19.                 cnt++;
  20.             }
  21.         }
  22.     }
  23.     cout << cnt * 2;
  24. }
  25.  
  26. int main()
  27. {
  28.     cin.tie(0) -> sync_with_stdio(false);
  29.     freopen("BAI2.INP", "r", stdin);
  30.     freopen("BAI2.OUT", "w", stdout);
  31.  
  32.     cin >> n;
  33.     for(int i = 1; i <= n; i++) cin >> a[i];
  34.     sort(a + 1, a + 1 + n);
  35.     if(n <= 3000) sub1();
  36.     else
  37.     {
  38.         long long cnt = 0;
  39.         for(int i = 1; i <= n; i++)
  40.         {
  41.             int j = lower_bound(a + 1, a + 1 + n, a[n] - a[i]) - a;
  42.             if(a[j] <= a[i]) break;
  43.             cnt += j - i;
  44.         }
  45.         cout << cnt * 2;
  46.     }
  47.     return 0;
  48. }
  49.  
Success #stdin #stdout 0.01s 5284KB
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https://ideone.com/eiY2xs
language:
C++ (gcc 8.3)
created:
2 days ago
visibility:
public

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