吃饱辣

#include <iostream>
#include <cstdio>
#include <map>
using namespace std;
map <int, int> mp;
int a[200005];
int main(){
	int n, m;
	cin >> n >> m;
	long long ct = 0;
	for(int i = 1; i <= n; i++){
		cin >> a[i];
		mp[a[i]]++;
	}for(int i = 1; i <= n; i++){
		if(a[i] >= m) ct += mp[a[i] - m];
	}
	cout << ct;
	

	return 0;
}

时间复杂度：$O(n\log_2a[i])$