A-B 数对(同向扫描

P1102 A-B 数对 - 洛谷 | 计算机科学教育新生态 (luogu.com.cn)

#include<bits/stdc++.h>
using namespace std;
const int N=200001;
#define int long long
int a[N];
signed main(){
	ios::sync_with_stdio(false);
	cin.tie(0),cout.tie(0);
	int n,c;
	cin>>n>>c;
	for(int i=1;i<=n;i++)cin>>a[i];
	sort(a+1,a+n+1);
	int ans=0;
    for(int i=1,j=1,k=1;i<=n;i++){
		while(a[j]+c<a[i]&&j<=n) j++;
		while(a[k]+c<=a[i]&&k<=n) k++;
	    if(a[j]+c==a[i])ans+=(k-j);
	}
	cout<<ans;
	return 0;
}