input: Write a function to find 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([-1, -5, -2, -9])==-1
  assert find_max([10])==10
  assert find_max([3, 3, 3, 3])==3
  assert find_max([2, 4, 6, 8, 10])==10
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