双指针

#include <bits/stdc++.h>
using namespace std;
int n, C;
int a[212345];
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> C;
    for (int i = 1; i <= n; i++)
        cin >> a[i];
    sort(a + 1, a + n + 1);
    long long ans = 0;
    int l = 1; // 第一个大于等于 a[i]+C 的位置
    int r = 1; // 第一个大于 a[i]+C 的位置
    for (int i = 1; i <= n; i++)
    {
        while (l <= n && !(a[l] >= a[i] + C))
            l++;
        while (r <= n && !(a[r] > a[i] + C))
            r++;
        ans += r - l;
    }
    cout << ans << "\n";
    return 0;
}

二分查找

#include <bits/stdc++.h>
using namespace std;
long long n, c, a[200005], ans;
int main()
{
    cin >> n >> c;
    for (int i = 1; i <= n; i++)
    {
        cin >> a[i];
    }
    sort(a + 1, a + n + 1);
    ans = 0;
    for (int i = 1; i <= n; i++)
    {
        ans += upper_bound(a + 1, a + n + 1, a[i] - c) -
               lower_bound(a + 1, a + n + 1, a[i] - c);
    }
    cout << ans << endl;
    return 0;
}