#896. 「雅礼集训 2018 Day1」折射（refract）

https://sjzezoj.com/submission/58303

S2OJ/896/896.cpp

@@ -0,0 +1,46 @@
+#include <iostream>
+#include <algorithm>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 6005;
+const int mod = 1e9 + 7;
+
+int n, f[N][2], ans;
+std::pair<int, int> a[N];
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> n;
+
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i].first >> a[i].second;
+    }
+
+    std::sort(a + 1, a + 1 + n);
+
+    f[1][0] = f[1][1] = 1;
+    for (int i = 2; i <= n; i++) {
+        f[i][0] = f[i][1] = 1;
+
+        for (int j = i - 1; j; j--) {
+            if (a[j].second > a[i].second) {
+                f[j][1] = (f[j][1] + f[i][0]) % mod;
+            } else {
+                f[i][0] = (f[i][0] + f[j][1]) % mod;
+            }
+        }
+    }
+
+    for (int i = 1; i <= n; i++) {
+        ans = (static_cast<long long>(ans) + f[i][0] + f[i][1] - 1) % mod;
+    }
+
+    cout << ans << endl;
+
+    return 0;
+}