input: Write a function to implement a binary search in a sorted list. The function should return the index of the target value if it exists in the list, otherwise return -1.
  assert binary_search([3, 4, 5, 6, 7, 8, 9], 8) == 5
  assert binary_search([3, 4, 5, 6, 7, 8, 9], 10) == -1
  assert binary_search([], 8) == -1
  assert binary_search([3, 4, 5, 6, 7, 8, 9], 3) == 0
  assert binary_search([3, 4, 5, 6, 7, 8, 9], 4) == 1
output: def binary_search(arr, target):
      left, right = 0, len(arr) - 1
      while left <= right:
          mid = (left + right) // 2
          if arr[mid] == target:
              return mid
          elif arr[mid] < target:
              left = mid + 1
          else:
              right = mid - 1
      return -1