CF1833F - Ira and Flamenco

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int Mod = 1e9 + 7;
const int N = 2e5 + 10;
int t, n, m, ans, c;
int a[N];
map<int, int> cnt;
int qpow(int x, int y) {
    if(y == 0) return 1;
    if(y == 1) return x;
    int t = qpow(x, y / 2);
    if (y & 1) return t * t % Mod * x % Mod;
    else return t * t % Mod;
}
signed main() {
    cin >> t;
    while (t--) {
        memset(a, 0, sizeof(a));
        cnt.clear();
        ans = 0, c = 1;
        cin >> n >> m;
        for (int i = 1; i <= n; i++) {
            cin >> a[i];
            cnt[a[i]]++;
        }
        sort(a + 1, a + 1 + n);
        n = unique(a + 1, a + 1 + n) - a - 1;
        for (int i = 1, j = 1; i <= n; ++i) {
            while (j <= n && a[j] < a[i] + m) c = c * (cnt[a[j]]) % Mod, ++j;
            if (j - i == m) ans += c, ans %= Mod;
            c = c * qpow(cnt[a[i]], Mod - 2) % Mod;
        }
        cout << ans << '\n';
    }
}