A-B 数对

https://www.luogu.com.cn/problem/P1102

include <bits/stdc++.h>

define ll long long

using namespace std;

const int N = 2e5 + 10;
int n , c;
int a[N];

int main ()
{
cin >> n >> c;
for(int i = 1 ; i <= n ; i ++) cin >> a[i];
sort(a + 1 , a + 1 + n);
int l = 1, r1 = 1 , r2 = 1;
ll ans = 0;
for(l = 1 ; l <= n ; l ++) {
while(r1 <= n && a[r1] - a[l] <= c) r1 ++;
while(r2 <= n && a[r2] - a[l] < c ) r2 ++;
if(a[r2] - a[l] == c && a[r1 - 1] - a[l] == c && r1 - 1 >= 1)
ans += r1 - r2;
}
cout << ans;
return 0;}