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_53/sol1.py

@@ -1,13 +1,11 @@
-#-.- coding: latin-1 -.-
-from __future__ import print_function
-from math import factorial
-'''
+# -.- coding: latin-1 -.-
+"""
 Combinatoric selections
 Problem 53
 
 There are exactly ten ways of selecting three from five, 12345:
 
-123, 124, 125, 134, 135, 145, 234, 235, 245, and 345
+    123, 124, 125, 134, 135, 145, 234, 235, 245, and 345
 
 In combinatorics, we use the notation, 5C3 = 10.
 
@@ -16,21 +14,37 @@ In general,
 nCr = n!/(r!(n−r)!),where r ≤ n, n! = n×(n−1)×...×3×2×1, and 0! = 1.
 It is not until n = 23, that a value exceeds one-million: 23C10 = 1144066.
 
-How many, not necessarily distinct, values of nCr, for 1 ≤ n ≤ 100, are greater than one-million?
-'''
+How many, not necessarily distinct, values of nCr, for 1 ≤ n ≤ 100, are greater
+than one-million?
+"""
+from __future__ import print_function
+from math import factorial
+
 try:
-	xrange		#Python 2
+    xrange  # Python 2
 except NameError:
-	xrange = range	#Python 3
+    xrange = range  # Python 3
+
 
 def combinations(n, r):
-	return factorial(n)/(factorial(r)*factorial(n-r))
+    return factorial(n) / (factorial(r) * factorial(n - r))
 
-total = 0
 
-for i in xrange(1, 101):
-	for j in xrange(1, i+1):
-		if combinations(i, j) > 1e6:
-			total += 1
+def solution():
+    """Returns the number of values of nCr, for 1 ≤ n ≤ 100, are greater than
+    one-million
 
-print(total)
+    >>> solution()
+    4075
+    """
+    total = 0
+
+    for i in xrange(1, 101):
+        for j in xrange(1, i + 1):
+            if combinations(i, j) > 1e6:
+                total += 1
+    return total
+
+
+if __name__ == "__main__":
+    print(solution())