E - BBQ Hard

https://atcoder.jp/contests/agc001/submissions/37074356

AtCoder/AGC001/E/E.cpp

@@ -0,0 +1,70 @@
+#include <iostream>
+#include <algorithm>
+#include <cstring>
+#include <iterator>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 2e5 + 5,
+          M = 2005;
+const int mod = 1e9 + 7;
+
+int n, m, a[N], b[N], inv[M << 2], fac[M << 2], inv_fac[M << 2], _f[M << 1][M << 1], *_pf[M << 1], **f, ans;
+
+inline int C(int n, int m) {
+    return static_cast<long long>(fac[n]) * inv_fac[m] % mod * inv_fac[n - m] % mod;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    fac[0] = 1;
+    for (int i = 1; i < M << 2; i++) {
+        fac[i] = static_cast<long long>(fac[i - 1]) * i % mod;
+    }
+
+    inv[0] = inv[1] = 1;
+    for (int i = 2; i < M << 2; i++) {
+        inv[i] = static_cast<long long>(mod - (mod / i)) * inv[mod % i] % mod;
+    }
+
+    inv_fac[0] = inv_fac[1] = 1;
+    for (int i = 2; i < M << 2; i++) {
+        inv_fac[i] = static_cast<long long>(inv_fac[i - 1]) * inv[i] % mod;
+    }
+
+    for (int i = 0; i < M << 1; i++) {
+        _pf[i] = _f[i] + M;
+    }
+
+    f = _pf + M;
+
+    cin >> n;
+
+    for (int i = 1; i <= n; i++) {
+        cin >> a[i] >> b[i];
+
+        f[-a[i]][-b[i]]++;
+    }
+
+    for (int i = -2000; i <= 2000; i++) {
+        for (int j = -2000; j <= 2000; j++) {
+            f[i][j] = (static_cast<long long>(f[i][j]) + f[i - 1][j] + f[i][j - 1]) % mod;
+        }
+    }
+
+    for (int i = 1; i <= n; i++) {
+        ans = (static_cast<long long>(ans) + f[a[i]][b[i]]) % mod;
+    }
+
+    for (int i = 1; i <= n; i++) {
+        ans = ((static_cast<long long>(ans) - C(2 * a[i] + 2 * b[i], 2 * a[i])) % mod + mod) % mod;
+    }
+
+    cout << (static_cast<long long>(ans) * inv[2] % mod) << endl;
+
+    return 0;
+}