Classroom Scheduling problem

This problem was asked by Snapchat.

Given an array of time intervals (start, end) for classroom lectures (possibly overlapping), find the minimum number of rooms required.

For example, given [(30, 75), (0, 50), (60, 150)], you should return 2

My Solution(Python):

def classRoomSchedule(classes):
    """
    :type classes: List[tuples(start, end)]
    :rtype: int
    """
    N = len(classes)
    if N==0:
        return 1
    if N==1:
        return 1
    start_times = [c[0] for c in classes]
    end_times = [c[1] for c in classes]
    start_times.sort()
    end_times.sort()
    i = 1
    j = 0
    num_classes = 1
    res = 1
    while i < N and j < N:
        if start_times[i] < end_times[j]:
            num_classes+=1
            if num_classes > res:
                res = num_classes
            i+=1
        else:
            num_classes-=1
            j+=1
    return res

import unittest
class TestClass(unittest.TestCase):
    def runTest(self):
        classes = [(30, 75), (0, 50), (60, 150)]
        self.assertEqual(classRoomSchedule(classes), 2)


if __name__=='__main__':
    unittest.main()