Added doctest and more explanation about Dijkstra execution. (#1014)

* Added doctest and more explanation about Dijkstra execution.

* tests were not passing with python2 due to missing __init__.py file at number_theory folder

* Removed the dot at the beginning of the imported modules names because 'python3 -m doctest -v data_structures/hashing/*.py' and 'python3 -m doctest -v data_structures/stacks/*.py' were failing not finding hash_table.py and stack.py modules.

* Moved global code to main scope and added doctest for project euler problems 1 to 14.

* Added test case for negative input.

* Changed N variable to do not use end of line scape because in case there is a space after it the script will break making it much more error prone.

* Added problems description and doctests to the ones that were missing. Limited line length to 79 and executed python black over all scripts.

* Changed the way files are loaded to support pytest call.

* Added __init__.py to problems to make them modules and allow pytest execution.

* Added project_euler folder to test units execution

* Changed 'os.path.split(os.path.realpath(__file__))' to 'os.path.dirname()'

project_euler/problem_20/sol1.py

@@ -1,27 +1,51 @@
-# Finding the factorial.
+"""
+n! means n × (n − 1) × ... × 3 × 2 × 1
+
+For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,
+and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
+
+Find the sum of the digits in the number 100!
+"""
+
+
 def factorial(n):
     fact = 1
-    for i in range(1,n+1):
+    for i in range(1, n + 1):
         fact *= i
     return fact
 
-# Spliting the digits and adding it.
+
 def split_and_add(number):
+    """Split number digits and add them."""
     sum_of_digits = 0
-    while(number>0):
+    while number > 0:
         last_digit = number % 10
         sum_of_digits += last_digit
-        number = int(number/10) # Removing the last_digit from the given number.
+        number = number // 10  # Removing the last_digit from the given number
     return sum_of_digits
 
-# Taking the user input.
-number = int(input("Enter the Number: "))
 
-# Assigning the factorial from the factorial function.
-factorial = factorial(number)
+def solution(n):
+    """Returns the sum of the digits in the number 100!
+    >>> solution(100)
+    648
+    >>> solution(50)
+    216
+    >>> solution(10)
+    27
+    >>> solution(5)
+    3
+    >>> solution(3)
+    6
+    >>> solution(2)
+    2
+    >>> solution(1)
+    1
+    """
+    f = factorial(n)
+    result = split_and_add(f)
+    return result
 
-# Spliting and adding the factorial into answer.
-answer = split_and_add(factorial)
 
-# Printing the answer.
-print(answer)
+if __name__ == "__main__":
+    print(solution(int(input("Enter the Number: ").strip())))