diff --git a/khj20006/202502/27 BOJ P3 Starting a Scenic Railroad Service.md b/khj20006/202502/27 BOJ P3 Starting a Scenic Railroad Service.md
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@@ -0,0 +1,95 @@
+```cpp
+
+#include <iostream>
+#include <vector>
+#include <algorithm>
+#include <set>
+#include <ext/pb_ds/assoc_container.hpp>
+#include <ext/pb_ds/tree_policy.hpp>
+using namespace std;
+using namespace __gnu_pbds;
+using ll = long long;
+
+int seg[262145][2]{};
+
+#define ordered_set tree<ll, null_type, less_equal<ll>, rb_tree_tag, tree_order_statistics_node_update>
+ordered_set os;
+
+void upt(int s, int e, int i, int n, int x) {
+	if (s == e) {
+		seg[n][x]++;
+		return;
+	}
+	int m = (s + e) >> 1;
+	if (i <= m) upt(s, m, i, n * 2, x);
+	else upt(m + 1, e, i, n * 2 + 1, x);
+	seg[n][x]++;
+}
+
+int find(int s, int e, int l, int r, int n, int x) {
+	if (l > r || l > e || r < s) return 0;
+	if (l <= s && e <= r) return seg[n][x];
+	int m = (s + e) >> 1;
+	return find(s, m, l, r, n * 2, x) + find(m + 1, e, l, r, n * 2 + 1, x);
+}
+
+int main() {
+	cin.tie(0)->sync_with_stdio(0);
+
+	int N;
+	cin >> N;
+
+	vector<vector<int>> in(100001), out(100001);
+	int H[200001]{};
+
+	vector<pair<int, int>> V;
+	for (int i = 1, a, b; i <= N; i++) {
+		cin >> a >> b;
+		in[a].push_back(i);
+		out[b].push_back(i);
+		V.emplace_back(a, b);
+		upt(1, 100000, a, 1, 0);
+		upt(1, 100000, b, 1, 1);
+
+	}
+
+	int s1 = 0;
+	sort(V.begin(), V.end(), [](auto a, auto b) -> bool {
+		if (a.second == b.second) return a.first > b.first;
+		return a.second < b.second;
+	});
+
+	for (int i=0;i<V.size();i++) {
+	    while(i<V.size()-1 && V[i].first == V[i+1].first){
+	        os.insert(V[i++].first);
+	    }
+	    int a = V[i].first, b = V[i].second;
+		int res = find(1, 100000, a, b-1, 1, 0) + find(1, 100000, a+1, b, 1, 1) - 1;
+
+        
+		res -= (os.size() - os.order_of_key(a));
+		os.insert(a);
+
+		s1 = max(s1, res);
+	}
+
+
+	set<int> S = { 0 };
+	int s2 = 0, mex = 0;
+	for (int x = 1; x <= 100000; x++) {
+		for (int i : out[x]) {
+			S.erase(H[i]);
+			mex = min(mex, H[i]);
+		}
+		for (int i : in[x]) {
+			while (S.count(mex)) mex++;
+			S.insert(H[i] = mex++);
+		}
+		s2 = max(s2, *S.rbegin());
+	}
+
+	cout << s1 << ' ' << s2;
+
+}
+
+```