input: Write a function to create a dictionary which contains a data structure in the form of [x:x^2] where x is the key and x^2 is the value. If the input list is empty, return an empty dictionary.
  assert dict_square([1, 2, 3, 4, 5])=={1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
  assert dict_square([])=={}
output: def dict_square(lst):
      return {x: x**2 for x in lst}