题解

#include <iostream>
using namespace std;

int main() {
    int n, sum;
    int a[10000];
    int b[10000] = {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;}