First and last train

A timetable lists departure times from earliest to latest. Several trains can leave at the same time, so a time can appear more than once. You are given the list times and a time t. Return a list [first, last]: the first index and the last index where t appears. If t is not in the list, return [-1, -1].

first_and_last([2, 4, 4, 7, 7, 7, 7, 9, 12], 7)   # [3, 6]
first_and_last([1, 3, 5, 8], 5)                  # [2, 2]
first_and_last([1, 3, 5, 8], 4)                  # [-1, -1]

Your search must stay fast even when t appears half a million times.

In the large tests, the list counts every value you read from it. A binary search stays well under the limit. Anything that goes through the whole list goes over it. This includes in, .index(), list(…), sorted(…) and slicing.

Limits

• Up to about 600,000 times. At most 60 reads on the large tests.
• Return a list of two integers.