input: def linear_search(arr: list, x) -> int:
      """Return the index of the first occurrence of x in arr, or -1 if x is not found.
      >>> linear_search([1, 2, 3, 4, 5], 3)
      2
      >>> linear_search([1, 2, 3, 4, 5], 6)
      -1
      """
output:     for i in range(len(arr)):
          if arr[i] == x:
              return i
      return -1