下面折半查找程序的时间复杂度为（    ）

def binary_search(arr, x):

    low = 0

    high = len(arr) - 1

    while low <= high:

            mid = (low + high) // 2

            if arr[mid] == x:

                  return mid

            elif arr[mid] > x:

                  high = mid - 1

            else:

                  low = mid + 1

      return -1