input: Write a function to convert a given list of integers into a dictionary where each integer is the key and the value is its corresponding string representation. If the 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[num] for num in lst if num in num_to_str}