Update factorial.c

Added the Factorial program in C.

Author: IamBisrutPyne

snippets/Basics/c/factorial.c

@@ -1,6 +1,34 @@
-# Title: Iterative factorial
-# Topic: Basics
-# Language: c
-# Example: see bottom
+// Metadata Header (MANDATORY)
+// -----------------------------
+// Program Title: Factorial Calculation (Iterative)
+// Author: IamBisrutPyne
+// Date: 2025-10-10
+//
+// Description: Computes the factorial of a non-negative integer using an iterative approach.
+//
+// Language: C
+//
+// Time Complexity: O(n)
+// Space Complexity: O(1)
+// -----------------------------
 
-// Iterative factorial - placeholder in c
+#include <stdio.h>
+
+// Calculates n! iteratively. Using long long for potentially larger results.
+long long factorial(int n) {
+    if (n < 0) {
+        return 0;
+    }
+    long long result = 1;
+    for (int i = 2; i <= n; i++) {
+        result *= i;
+    }
+    return result;
+}
+
+int main() {
+    int num = 10;
+    printf("Factorial of %d is: %lld\n", num, factorial(num));
+    // Output: Factorial of 10 is: 3628800
+    return 0;
+}