rewrite the algorithm from scratch (#1351)

maths/factorial_python.py

@@ -1,19 +1,21 @@
-"""Python program to find the factorial of a number provided by the user."""
+def factorial(input_number: int) -> int:
+    """
+        Non-recursive algorithm of finding factorial of the
+        input number.
+        >>> factorial(1)
+        1
+        >>> factorial(6)
+        720
+        >>> factorial(0)
+        1
+    """
 
-# change the value for a different result
-NUM = 10
-
-# uncomment to take input from the user
-# num = int(input("Enter a number: "))
-
-FACTORIAL = 1
-
-# check if the number is negative, positive or zero
-if NUM < 0:
-    print("Sorry, factorial does not exist for negative numbers")
-elif NUM == 0:
-    print("The factorial of 0 is 1")
-else:
-    for i in range(1, NUM + 1):
-        FACTORIAL = FACTORIAL * i
-    print("The factorial of", NUM, "is", FACTORIAL)
+    if input_number < 0:
+        raise ValueError('Input input_number should be non-negative')
+    elif input_number == 0:
+        return 1
+    else:
+        result = 1
+        for i in range(input_number):
+            result = result * (i + 1)
+    return result