Python

This was the easier one for me out of the first 3 days. Cleaned up my solution before posting for better readability:

# get joltage of picked batteries
def get_joltage(batteries_picked: list[int]):
    bank_joltage = 0
    for batt in batteries_picked:
        bank_joltage = bank_joltage * 10 + batt
    return bank_joltage

# get maximum joltage of a bank
def get_bank_joltage(bank: str, pick_limit = 2) -> int:
    # pick first <pick_limit> batteries
    batteries_picked = [int(bank[i]) for i in range(pick_limit)]
    max_joltage = get_joltage(batteries_picked)

    # iterate over remaining batteries
    for i in range(pick_limit, len(bank)):
        batt = int(bank[i])        
        # we add batt for selection consideration
        batteries_picked.append(batt)
        # If all batteries are in descending order and batt is the lowest, 
        #   we will eventually discard batt
        to_discard = pick_limit

        # However if not, we discard the leftmost MSB battery which has lower joltage than its successor
        #   and shift all batteries left with batt added at the end.
        # This guarantees that we keep the maximum lexicographical order of picked batteries
        #   regardless of batt's value.
        for i in range(pick_limit):
            if batteries_picked[i] < batteries_picked[i+1]:
                to_discard = i
                break
        batteries_picked.pop(to_discard)

        # update max_joltage, it may have increased
        max_joltage = max(max_joltage, get_joltage(batteries_picked))

    return max_joltage

# part 1 asserts
assert get_bank_joltage("987654321111111", pick_limit=2) == 98
assert get_bank_joltage("811111111111119", pick_limit=2) == 89
assert get_bank_joltage("234234234234278", pick_limit=2) == 78
assert get_bank_joltage("818181911112111", pick_limit=2) == 92

# part 2 asserts
assert get_bank_joltage("987654321111111", pick_limit=12) == 987654321111
assert get_bank_joltage("811111111111119", pick_limit=12) == 811111111119
assert get_bank_joltage("234234234234278", pick_limit=12) == 434234234278
assert get_bank_joltage("818181911112111", pick_limit=12) == 888911112111

# get total joltage of a set of banks
def solve(data: str, pick_limit = 2):
    total_joltage = 0
    for bank in data.splitlines():
        total_joltage += get_bank_joltage(bank, pick_limit)
    return total_joltage

# asserts for sample data
sample = """987654321111111
811111111111119
234234234234278
818181911112111"""
assert solve(sample, pick_limit=2) == 357               # part 1
assert solve(sample, pick_limit=12) == 3121910778619    # part 2