input: Write a python function to perform a linear search on a list of integers.
  assert linearSearch([1, 2, 3, 4], 3) == 2
  assert linearSearch([10, 20, 30, 40, 50], 25) == -1
  assert linearSearch([5, 15, 25, 35, 45], 45) == 4
output: def linearSearch(arr, target):
      for i in range(len(arr)):
          if arr[i] == target:
              return i
      return -1