input: Write a python function to perform a linear search to find the index of a given key in a list.
  assert linearSearch([2, 8, 9, 5, 15, 12, 4], 12) == 5
  assert linearSearch([2, 8, 9, 5, 15, 12, 4], 3) == -1
  assert linearSearch([1, 3, 5, 7, 9, 11, 13], 7) == 3
output: def linearSearch(arr, key):
      for index, value in enumerate(arr):
          if value == key:
              return index
      return -1