Modified Blackjack

Source: May 2025, Task 2 (DTU 02002/02003). Study solution — not an official answer key.

Problem

You have a stack of card values (list of ints, first element is the top). Remove n cards from the top so the next k cards sum exactly to 21.

Write modified_blackjack(cards) returning (n, k). If none exists, return (-1, -1). If several exist, choose the smallest n.

>>> modified_blackjack([5, 2, 3, 9, 7, 6, 2, 4])
(1, 4)

Approach

Try each possible n from 0 upward. From the new top, add consecutive cards until the sum hits 21 (success) or exceeds 21 (try next n).

Solution

def modified_blackjack(cards):
    for n in range(len(cards)):
        total = 0
        for k in range(1, len(cards) - n + 1):
            total += cards[n + k - 1]
            if total == 21:
                return (n, k)
            if total > 21:
                break
    return (-1, -1)