題目傳送門

#include <bits/stdc++.h>

using namespace std;
typedef long long LL;
const int N = 200010;
int a[N];
int n, c;
LL cnt; //累加和不加long long 死翹翹

int main() {
    //輸入
    cin >> n >> c;
    for (int i = 1; i <= n; i++) cin >> a[i];

    //排序
    sort(a + 1, a + 1 + n);

    //遍歷每一個數字
    for (int i = 1; i < n; i++) {
        //每個數字可能都是多個，我們不能漏掉每個數字
        int left = 0, right = 0;

        //左端點
        int l = 1, r = n, k = a[i] + c;//k為目標值
        while (l < r) {
            int mid = (l + r) >> 1;
            if (a[mid] >= k)r = mid;
            else l = mid + 1;
        }
        if (a[l] == k)left = l;

        //右端點
        l = 1, r = n;
        while (l < r) {
            int mid = (l + r + 1) >> 1;
            if (a[mid] <= k)l = mid;
            else r = mid - 1;
        }
        if (a[l] == k) right = r;

        //左右端點差就是個數,其實left如果不是0，那麼right肯定不是0
        if (left && right) cnt += right - left + 1;
    }
    //輸出結果
    printf("%lld", cnt);
    return 0;
}