Show the best price still to come

A pricing dashboard shows, next to each day in a price history, the best price still to come.

Write best_price_ahead(prices) that returns, for each day, the highest price on any later day.

Input and output:

• prices: a list of 0 to 100,000 daily prices, each an integer from 0 to 1,000,000.
• Returns a list of integers of the same length, one per day, in the same order.

Rules:

• The value for day i is the largest price among the days after day i. Day i itself does not count.
• The last day has no later days, so its value is -1.
• An empty list returns an empty list.