input: Write a function to return the maximum number in a list. If the list is empty, return 0.
  assert find_max([1, 15, 20, 6])==20
  assert find_max([9, 3, 7, 1])==9
  assert find_max([5])==5
  assert find_max([])==0
output: def find_max(lst):
      if not lst:
          return 0
      return max(lst)