Hi, here's your problem today. This problem was recently asked by Google:

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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:
<pre lang=json>
{
  'CSC300': ['CSC100', 'CSC200'], 
  'CSC200': ['CSC100'], 
  'CSC100': []
}
</pre>

This input should return the order that we need to take these courses:
 ['CSC100', 'CSC200', 'CSCS300']

Here's your starting point:

<pre lang=py>
def courses_to_take(course_to_prereqs):
  # Fill this in.

courses = {
  'CSC300': ['CSC100', 'CSC200'], 
  'CSC200': ['CSC100'], 
  'CSC100': []
}

print courses_to_take(courses)
# ['CSC100', 'CSC200', 'CSC300']
</pre>