Added recursive factorial calculator (#1326)

math/factorial_recursion.c

@@ -0,0 +1,40 @@
+#include <stdio.h>
+#include <assert.h>
+#include <stdbool.h>
+
+// finds factorial of a given inegral input. Assumes 0!=1.
+int factorial(int n)
+{
+    if (n == 0)
+    {
+        return 1; // base case
+    }
+    return n * factorial(n - 1); // based on the fact that n*(n-1)! = n!
+}
+
+bool check_arrays(int *a, int *b, int pos)
+{
+    if (factorial(a[pos]) == b[pos])
+        return true;
+    else
+        return false;
+}
+
+static void tests()
+{
+    /* tests have a few integral factorials*/
+    int tests[] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11};
+    int expected[] = {1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800, 39916800};
+
+    for (int i = 0; i < 11; i++)
+    {
+        assert(check_arrays(tests, expected, i) == true);
+    }
+    printf("All test cases passed !\n");
+}
+
+int main()
+{
+    tests();
+    return 0;
+}