只有十二行。 一行流算是被我玩明白了

思路：既然这是四个数，那就2+2=4，求和2，计数也是2

#include<bits/stdc++.h> using namespace std; int main() { int n,sum; int a[10005]; int b[10005]={0}; long long count=0; cin>>n>>sum; for(int i=0;i<n;i++){ cin>>a[i]; } for(int i=0;i<n;i++){ for (int j=0;j<n;j++){ b[a[i]+a[j]]; } } for (int i=0;i<n;i) { for (int j=0;j<n;j++){ if (sum-a[i]-a[j]>0){ count+=b[sum-a[i]-a[j]]; } } } cout<<count; return 0; }

#include <iostream> using namespace std; int n, sum; int a[10000]; int b[10000]; int main() { long long count = 0; cin >> n >> sum; for (int i = 0; i < n; i++){ cin >> a[i]; } for (int i = 0; i < n; i++){ for (int j = 0; j < n; j++){ b[a[i] + a[j]]; } } for (int i = 0; i < n; i){ for (int j = 0; j < n; j++){ if (sum - a[i] - a[j] > 0){ count += b[sum - a[i] - a[j]]; } } } cout << count; return 0; }

#include <iostream> using namespace std; int main() { int n, sum; int a[10000]; int b[10000] = {0}; long long cnt = 0; }

#include <iostream> using namespace std; int main() { int n, sum; int a[10000]; int b[10000] = {0}; long long cnt = 0; }