feat: add combinatorics ncr compress snippet

Author: LuchoBazz

combinatorics/combinatorics_ncr_compress.cpp

@@ -0,0 +1,40 @@
+const int64_t md = 1e9 + 7;
+
+int64_t fastpow(int64_t a, int64_t b, const int64_t &mod) {
+    assert(0 <= b);
+    if (b == 0)
+        return 1;
+    int64_t half = fastpow(a, b / 2, mod);
+    int64_t answer = half * half % mod;
+    if (b & 1)
+        answer = answer * a % mod;
+    return answer;
+}
+
+int64_t inv(int64_t a, const int64_t &mod) { return fastpow(a, mod - 2, mod); }
+
+const int MXF = 1e6;
+
+int64_t fact[MXF];
+
+void build_factorial() {
+    fact[0] = 1;
+    for (int i = 1; i < MXF; ++i) {
+        fact[i] = fact[i - 1] * i % md;
+    }
+}
+
+int64_t nCr(int n, int r) {
+    if (n < r)
+        return 0;
+    return fact[n] * inv(fact[n - r] * fact[r] % md, md) % md;
+};
+
+// auto nCr = [&](int n, int r) -> int64_t {
+//     if (n < r)
+//         return 0;
+//     return fact[n] * inv(fact[n - r] * fact[r] % md, md) % md;
+// };
+
+// at main()
+// build_factorial();