input: Write a function to return the maximum number in a list.
  def find_max(nums):
      '''Returns the maximum number in the list.'''
      max_val = nums[0]
      for num in nums:
          if num > max_val:
              max_val = num
      
      return max_val
  assert find_max([1, 15, 20, 6])==20
  assert find_max([5, 10, 15, 20])==20
  assert find_max([1])==1
  assert find_max([3, 3, 3, 3])==3
  assert find_max([-1, -5, -3, -7])==-1
output: def find_max(nums):
      '''Returns the maximum number in the list.'''
      max_val = nums[0]
      for num in nums:
          if num > max_val:
              max_val = num
      
      return max_val