input: Write a function to perform a linear search on an array and return the index of the given value if found, or -1 if not found.
  assert linearSearch([5, 3, 7, 2], 7) == 2
  assert linearSearch([5, 3, 7, 2], 8) == -1
  assert linearSearch([], 5) == -1
  assert linearSearch([1, 1, 1], 1) == 0
output: def linearSearch(arr, value):
      for i in range(len(arr)):
          if arr[i] == value:
              return i
      return -1