[20251024] BOJ / P4 / 홍준이의 교집합 / 권혁준

Author: oncsr

khj20006/202510/24 BOJ P4 홍준이의 교집합.md

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+```cpp
+#include <bits/stdc++.h>
+using namespace std;
+using ll = long long;
+
+ll N, K;
+const ll MOD = 1e9 + 7;
+vector<pair<ll, int>> events;
+ll c[200001]{};
+
+ll power(ll a, ll b) {
+	if (b == 0) return 1;
+	if (b == 1) return a % MOD;
+	ll h = power(a, b >> 1);
+	h = (h * h) % MOD;
+	return (b & 1) ? h * a % MOD : h;
+}
+
+int main() {
+	cin.tie(0)->sync_with_stdio(0);
+
+	cin >> N >> K;
+
+	c[K] = 1;
+	for (int x = 1; K + x <= N; x++) c[K + x] = c[K + x - 1] * (K + x) % MOD * power(x, MOD - 2) % MOD;
+
+	for (int i = 0, l, r; i < N; i++) {
+		cin >> l >> r;
+		events.emplace_back(l, 1);
+		events.emplace_back(r + 1, -1);
+	}
+
+	sort(events.begin(), events.end());
+	
+	ll ans = 0, prev = MOD, cnt = 0;
+	for (int i = 0; i < N * 2;) {
+		ll prevCnt = cnt, cur = events[i].first;
+		while (i < N * 2 && events[i].first == cur) cnt += events[i++].second;
+		if (prev != MOD) ans = (ans + (cur - prev) * c[prevCnt]) % MOD;
+		prev = cur;
+	}
+	cout << ans;
+
+}
+```