input: Write a function to find the maximum number in a list. The function should iterate through the list and return the largest number found.
  
  assert find_max([1, 15, 20, 6])==20
  assert find_max([-1, -5, -10, -3])==-1
  assert find_max([100])==100
  assert find_max([])==None
output: def find_max(lst):
      if not lst:
          return None
      max_num = lst[0]
      for num in lst:
          if num > max_num:
              max_num = num
      return max_num