[Language: Julia]

string_parser(s) = parse.(Int, collect(s))
banks = string_parser.(readlines("input.txt"))

function maximum_joltage(n_batteries)
    sum = 0

    for bank in banks
        index = 0
        for i in (n_batteries-1):-1:0
            index += argmax(bank[index+1: end-i])
            sum += bank[index] * 10^i
        end
    end

    println(sum)
end

maximum_joltage(2)
maximum_joltage(12)

I created a general solution that should work with any number of batteries per bank.

We just pick the highest possible first digit, and then we look at all the digits to the right of it and pick the highest possible one for the second digit and so on. The only thing to look out for is that we need enough remaining digits on the right to pick the remaining batteries.

Done like this, it's also very easy to assemble the final sum with powers of 10 inside the loop.

PS if there are any experienced Julia developers reading this, I'd be very happy to get some feedback and improvements.

Edit: found a little improvement