done with basic calc

exercises/02_basic_calculator/07_for/src/lib.rs

@@ -1,6 +1,13 @@
 // Rewrite the factorial function using a `for` loop.
 pub fn factorial(n: u32) -> u32 {
-    todo!()
+    if n == 0 || n == 1 {
+        return 1;
+    }
+    let mut prod = 1;
+    for i in 2..=n {
+        prod = prod * i;
+    }
+    prod
 }
 
 #[cfg(test)]