input: Write a python function to find the second largest number in a list.
  assert second_largest_num([2, 1, 4, 5]) == 4
  assert second_largest_num([10, 20, 20, 4, 5]) == 10
  assert second_largest_num([1, 1, 1, 1]) == 0
output: def second_largest_num(lst):
      if len(lst) < 2:
          return 0
      unique_lst = list(set(lst))
      if len(unique_lst) < 2:
          return 0
      unique_lst.sort()
      return unique_lst[-2]