Add solution for challenge #26

Author: Mauricio Klein

Diff for: README.md

@@ -25,3 +25,4 @@
 - [x] [Challenge 23: Invert a binary tree](challenge-23/)
 - [x] [Challenge 24: Move zeros](challenge-24/)
 - [x] [Challenge 25: Falling Dominoes](challenge-25/)
+- [x] [Challenge 26: Course Pre-requisites](challenge-26/)

Diff for: challenge-26/Makefile

@@ -0,0 +1,4 @@
+test:
+	python test_*.py
+
+.PHONY: test

Diff for: challenge-26/README.md

@@ -0,0 +1,24 @@
+# Course Pre-requisites
+
+This problem was asked by Google.
+
+## Description
+
+You are given a hash table where the key is a course code, and the value is a list of all the course codes that are prerequisites for the key. 
+
+Return a valid ordering in which we can complete the courses. 
+
+If no such ordering exists, return NULL.
+
+## Example
+
+```
+Input:
+{
+  'CSC300': ['CSC100', 'CSC200'], 
+  'CSC200': ['CSC100'], 
+  'CSC100': []
+}
+
+Output: ['CSC100', 'CSC200', 'CSC300']
+```

Diff for: challenge-26/__init__.py

@@ -0,0 +1,38 @@
+def courses_to_take(input):
+  """
+  Time complexity: O(n) (we process each course only once)
+  Space complexity: O(n) (array to store the result)
+  """
+  # Normalize the dependencies, using a set to track the
+  # dependencies more efficiently
+  course_with_deps = {}
+  to_take = []
+
+  for course, deps in input.items():
+    if not deps:
+      # Course with no dependencies:
+      # candidate to start the search
+      to_take.append(course)
+    else:
+      course_with_deps[course] = set(deps)
+
+  result = []
+  while to_take:
+    course = to_take.pop()
+
+    # Add course to journey
+    result.append(course)
+
+    # Iterate through courses and remove this course from
+    # dependencies
+    for prereq_course, prereq_deps in course_with_deps.items():
+      if course in prereq_deps:
+        prereq_deps.remove(course)
+      
+      if not prereq_deps:
+        # Course has all the dependencies solved:
+        # add to the "to_take" queue
+        to_take.append(prereq_course)
+        del course_with_deps[prereq_course]
+
+  return result if len(result) == len(input) else None

Diff for: challenge-26/solver.py

@@ -0,0 +1,26 @@
+import unittest
+from solver import courses_to_take
+
+class TestSolver(unittest.TestCase):
+  def test_courses_to_take(self):
+    self.assertEqual(
+      courses_to_take({
+        'CSC300': ['CSC100', 'CSC200'], 
+        'CSC200': ['CSC100'], 
+        'CSC100': []
+      }),
+      ['CSC100', 'CSC200', 'CSC300']
+    )
+
+    self.assertEqual(
+      courses_to_take({
+        'CSC400': ['CSC300', 'CSC200'], 
+        'CSC300': ['CSC200'],
+        'CSC200': ['CSC300', 'CSC100'],
+        'CSC100': []
+      }),
+      None
+    )
+
+if __name__ == "__main__":
+    unittest.main()