input: Write a function to optimize the given Python program to find the maximum number in a list. The function should iterate through the list and return the maximum number found.
  
  def max_num(list):
    if not list:
      return None
    max_num = list[0]
  
    for num in list:
      if num > max_num:
        max_num = num
    
    return max_num
  
  assert max_num([9, 2, 5, 6, 4, 1])==9
  assert max_num([-1, -2, -5, -6, -4, -10])==-1
  assert max_num([7])==7
  assert max_num([])==None
output: def max_num(list):
    if not list:
      return None
    max_num = list[0]
    for num in list:
      if num > max_num:
        max_num = num
    return max_num