NcR added

Author: rachitiitr

Library/Math/NcR.cpp

@@ -0,0 +1,40 @@
+#include<bits/stdc++.h>
+using namespace std;
+const int mod = 1'000'000'007;
+
+int mpow(int base, int exp) {
+  int result = 1;
+  while (exp > 0) {
+    if (exp & 1) result = ((long long)result * base) % mod;
+    base = ((long long)base * base) % mod;
+    exp >>= 1;
+  }
+  return result;
+}
+
+namespace NcR {
+    int fact[200005],ifact[200005];
+    int get(int n, int r) {
+        if(n<r||r<0||n<0) return 0;
+        return (((fact[n]*1LL*ifact[r])%mod)*1LL*ifact[n-r])%mod;
+    }
+
+    void init() {
+        fact[0]=1;
+        for(int i = 1; i <= 200000; i++) {
+            fact[i] = (fact[i-1] * 1LL * i)%mod;
+        }
+
+        ifact[200000] = mpow(fact[200000], mod-2);
+        for(int i=199999; i>=0; i--) {
+            ifact[i] = (ifact[i+1] * 1LL * (i+1))%mod;
+        }
+    }
+};
+
+int main() {
+
+    NcR::init();
+    cout << NcR::get(4, 2) << endl;
+    return 0;
+}