day 2 commit & changed day 1 filenames

D2_factorial.cpp

@@ -0,0 +1,19 @@
+#include <iostream>
+
+int factorial(int n) {
+    if (n <= 0) {
+        return -1;
+    } else if (n > 1) {
+        return n * factorial(n - 1); //n * n-1 * n-2....
+    } else return 1;
+}
+
+int main() {
+    int n;
+
+    std::cout << "Enter n to do n!: ";
+    std::cin >> n;
+
+    std::cout << "Ans: " << factorial(n) << std::endl;
+    return 0;
+}