refactoring: ending-permutation|combination

src/main/c/Calculator/calculator.c

@@ -128,6 +128,17 @@ double factorial(double num) {
         return num * factorial(num - 1);
     }
 }
+double permutation(int n, int r) {
+    double result = 1;
+    for (int i = 0; i < r; i++) {
+        result *= (n - i);
+    }
+    return result;
+}
+
+double combination(int n, int r) {
+    return permutation(n, r) / factorial(r);
+}
 
 int getIntInput(const char *message) {
     int input;