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  1. #include <bits/stdc++.h>
  2.  
  3. using namespace std;
  4.  
  5. int const NMAX = 1e5;
  6. int const MMAX = 1e6;
  7. int n, s;
  8. int freq[1 + MMAX];
  9. int pre[1 + MMAX];
  10.  
  11. int main() {
  12.     cin >> n >> s;
  13.     int selfPairs = 0;
  14.     for(int i = 1; i <= n; i++) {
  15.         int x; cin >> x;
  16.         freq[x]++;
  17.         if(2*x <= s) selfPairs++;
  18.     }
  19.     for(int i = 1; i <= MMAX; i++) {
  20.         pre[i] = pre[i-1] + freq[i];
  21.     }
  22.     int ans = 0;
  23.     for(int i = 1; i <= s; i++) {
  24.         ans += freq[i] * pre[s-i];
  25.     }
  26.     ans -= selfPairs;
  27.     ans /= 2;
  28.     cout << ans << "\n";
  29. }
Success #stdin #stdout 0.01s 9044KB
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stdin
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4 8
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stdout
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4
https://ideone.com/pI7i5c
language:
C++14 (gcc 8.3)
created:
5 days ago
visibility:
public

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