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- #include <bits/stdc++.h>
- #define int int64_t
- #define F first
- #define S second
- using namespace std;
- int n,k;
- vector<int> a;
- unordered_map<int,int32_t> m;
- int32_t col[10'000'500];
- int solve1()
- {
- int ans=0;
- sort(a.begin(),a.end());
- for(int32_t i=2;i<n-1;i++){
- for(int32_t j=i+1;j<n;j++){
- if(a[i]+a[j]>=k) continue;
- m[a[i]+a[j]]++;
- }
- }
- for(int32_t i=1;i<n;i++){
- for(int32_t j=0;j<i;j++){
- int sum=a[i]+a[j];
- if(k-sum<=0) break;
- ans+=m[k-sum];
- }
- for(int32_t j=i+2;j<n;j++){
- if(a[i+1]+a[j]>=k) break;
- m[a[i+1]+a[j]]--;
- }
- }
- return ans;
- }
- int solve2()
- {
- int ans=0;
- for(int32_t i=2;i<n-1;i++){
- for(int32_t j=i+1;j<n;j++){
- col[a[i]+a[j]]++;
- }
- }
- for(int32_t i=1;i<n;i++){
- for(int32_t j=0;j<i;j++){
- int sum=a[i]+a[j];
- if(k-sum<0) continue;
- ans+=col[k-sum];
- }
- for(int32_t j=i+2;j<n;j++){
- col[a[i+1]+a[j]]--;
- }
- }
- return ans;
- }
- int32_t main()
- {
- ios_base::sync_with_stdio(false);
- cin.tie(0); cout.tie(0);
- cin>>n>>k; int mx=0;
- for(int i=0;i<n;i++){
- int x; cin>>x; if(x>=k) continue;
- a.push_back(x);
- mx=max(mx,x);
- }
- n=a.size();
- if(mx<=5'000'000){
- cout<<solve2()<<endl;
- }else{
- cout<<solve1()<<endl;
- }
- }
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