input: Write a function to perform a linear search on a given array to find the position of a specified search item. If the item is found, return its position; otherwise, return -1. If the array is empty, return -1.
  assert linearSearch("bookstore")==2
  assert linearSearch("hospital")==1
  assert linearSearch("mall")==3
  assert linearSearch("theatre")==4
  assert linearSearch("hamlet")==-1
  assert linearSearch("")==-1
output: def linearSearch(arr):
      if not arr:
          return -1
      for i in range(len(arr)):
          if arr[i] == 'o':
              return i
      return -1