input: Write a python function to sort a list of strings using bubble sort algorithm.
  assert string_sort(["cat", "dog", "elephant", "apple"]) == ["apple", "cat", "dog", "elephant"]
  assert string_sort(["banana", "apple", "cherry"]) == ["apple", "banana", "cherry"]
  assert string_sort(["zebra", "monkey", "aardvark"]) == ["aardvark", "monkey", "zebra"]
output: def string_sort(arr):
      n = len(arr)
      for i in range(n):
          for j in range(0, n-i-1):
              if arr[j] > arr[j+1]:
                  arr[j], arr[j+1] = arr[j+1], arr[j]
      return arr