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  1. #include <bits/stdc++.h>
  2. using namespace std;
  3.  
  4. int main() {
  5. int n,target;
  6. cin>>n>>target;
  7.  
  8. vector<int> numbers(n);
  9.  
  10. for(auto& number: numbers){
  11. cin>>number;
  12. }
  13.  
  14. int answer = 0;
  15.  
  16. unordered_map<int,int> freq;
  17.  
  18. for(int i=0; i<n; i++){
  19.  
  20. int reqNum1 = target - numbers[i];
  21. int reqNum2 = -target - numbers[i];
  22.  
  23. if(freq.find(reqNum1) != freq.end())answer += freq[reqNum1];
  24. if(freq.find(reqNum2) != freq.end())answer += freq[reqNum2];
  25.  
  26. freq[numbers[i]]++;
  27. }
  28.  
  29. cout<<answer;
  30.  
  31. return 0;
  32. }
Success #stdin #stdout 0s 5312KB
stdin
5 2
10 5 1 3 1
stdout
1