input: Write a python function to find the median of a list of numbers.
  assert MedianFinder().find_median([1, 3, 5]) == 3
  assert MedianFinder().find_median([1, 2, 3, 4, 5, 6]) == 3.5
  assert MedianFinder().find_median([7, 8, 9, 4, 5]) == 7
output: class MedianFinder:
      def find_median(self, nums):
          nums.sort()
          n = len(nums)
          if n % 2 == 1:
              return nums[n // 2]
          else:
              mid1 = n // 2
              mid2 = mid1 - 1
              return (nums[mid1] + nums[mid2]) / 2