input: Write a function to perform linear search on a given array and return the position of the item if found, otherwise return -1. If the array is empty, return -1.
  assert linearSearch('school') == 0
  assert linearSearch('mall') == 3
  assert linearSearch('library') == -1
  assert linearSearch('') == -1
  assert linearSearch([]) == -1
output: def linearSearch(arr):
      if not arr:
          return -1
      for i, item in enumerate(arr):
          if item == 'school':
              return i
      return -1