input: Write a function that uses binary search to efficiently find the index of a target value in a sorted list of numbers. If the target value is found, return its index; otherwise, return -1.
  
  assert binary_search([3, 4, 5, 6, 7, 8, 9], 5) == 2
  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([], 5) == -1
  assert binary_search([1], 1) == 0
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