P1102 A-B 数对

#include<bits/stdc++.h>
using namespace std;
int main(){
	int n,c;
	cin>>n>>c;
	vector<int>p;
	p.push_back(-1);
	for(int i=1;i<=n;i++){
		int a;
		cin>>a;
		p.push_back(a);
	}
	long long ans=0;
	sort(p.begin(),p.end());
	for(int i=1;i<=n;i++){
		int cnt1=0,cnt2=0;
		int k=p[i];
		int l=i+1,r=n;
		while(l<r){
			int mid=(l+r)/2;  //靠左 
			if(p[mid]-k>=c)r=mid;
			else l=mid+1;
		}
		if(p[l]-k==c)cnt1=l;
		else continue;
		l=cnt1-1;
		r=n;
		while(l<r){
			int mid=(l+r+1)/2;// 靠右 
			if(p[mid]<=p[cnt1])l=mid;
			else r=mid-1;
		}
		ans+=r-cnt1+1;
	}
	cout<<ans;
}