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  1. #include<bits/stdc++.h>
  2. using namespace std;
  3.  
  4. #define int long long
  5.  
  6. const int N = 4e5 + 5;
  7. int n , k;
  8. int a[N];
  9. int cnt[N];
  10.  
  11. main()
  12. {
  13.     ios::sync_with_stdio(0);
  14.     cin.tie(0);
  15.     cin >> n >> k;
  16.     for(int i = 1 ; i <= n ; i++)
  17.         cin >> a[i];
  18.     int ans = 0;
  19.     /// ay + az = k - ax
  20.  
  21.     for(int y = 1 ; y <= n ; y++)
  22.     {
  23.         for(int z = y + 1 ; z <= n ; z++)
  24.         {
  25.             ans += cnt[a[y] + a[z] + (int)2e5];
  26.         }
  27.         cnt[k - a[y] + (int)2e5]++;
  28.     }
  29.     cout << ans;
  30.  
  31.     /// 0 <= a[y] + a[z] + 2e5 <= 4e5
  32.     /// 0 <= k - ay + 2e5      <= 4e5
  33.  
  34.  
  35.  
  36. }
Success #stdin #stdout 0.01s 5272KB
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https://ideone.com/VF79jQ
language:
C++14 (gcc 8.3)
created:
16 days ago
visibility:
public

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