题解 | 数对计数

#include <iostream>
#include <set>
#include <vector>
#include <algorithm>
using namespace std;

int main() {// 两层for循环不要用了，会超时，我试过了
    int n,c;
    cin >> n >> c;

    multiset<int> setANums;//多重集合，用来算重复元素的count
    int count = 0;
    for(int i = 0; i < n; i++) {
        int tmpNum = 0;
        cin >> tmpNum;
        setANums.insert(tmpNum);
    }

    for (auto num : setANums) {
        count += setANums.count(c+num);//举例，num = 1，c+num = 3，3在多重set中出现了两次，count+2
    }
    cout << count << endl;
}
// 64 位输出请用 printf("%lld")