input: Write a function to convert a given list into a dictionary where each element of the list is a key and its corresponding value is a string representation of that number. If the input list is empty, return an empty dictionary.
  assert list_to_dict([1, 2, 3, 4])=={1: 'one', 2: 'two', 3: 'three', 4: 'four'}
  assert list_to_dict([])=={}
output: def list_to_dict(lst):
      num_to_str = {1: 'one', 2: 'two', 3: 'three', 4: 'four'}
      return {num: num_to_str.get(num, str(num)) for num in lst}