68 lines
2.1 KiB
Plaintext
68 lines
2.1 KiB
Plaintext
Day 10 Notes
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+--------+
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| Part 1 |
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+--------+
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$ elixir day10part1.exs
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2450
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Thoughts:
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Noticed that the questions rely on you finding some insight into the data, rather than just
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implementing exactly what the instructions say.
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In this case, we just need to sort the data, and compute the difference between all the pairs.
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+--------+
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| Part 2 |
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+--------+
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$ elixir day10part2.exs
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32396521357312
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Thoughts:
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Fudge me this took me a long time (hours, including a walk in the park...)
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I spent far too long trying to find a mathematical answer based on combinations, and went down
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another rabbit hole identifying the "removable" items and then trying to calculate the answer
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based on that information.
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Turns out the trick I was missing is graph theory.
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Finally, after drawing out the example on paper, I realised that the number of combinations
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forms a directed graph, where the weight of each node inherits the weight of all its parents.
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For the example case (excuse my ascii drawing skills):
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0(1)-1(1)-4(1)-5(1)-|
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\ \ | |
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\ 6(2) |
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\ | |
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7(4)-|
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10(4)-11(4)
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\ |
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12(8)-18(8)-19(8)
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So just need to make an algorithm that builds this graph, by iterating the sorted list
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and checking the next 3 elements, keeping track of the weights as we go. It's actually simpler
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than that, because we don't need to keep the connections around, just the weightings for each node.
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Finally, the weight of the max joltage adapter is the answer.
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** Update **
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I added day10part2-refactored.exs, which contains a commented and arguably easier to understand
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version that uses Enum.reduce instead of so much recursion. However, in benchmarks it was
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negligibly (~1 microsecond) slower, so I decided to leave the original version in place.
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+------------------+
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| Overall Thoughts |
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+------------------+
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Part 1 was very easy, part 2 was fiendish. Glad this wasn't an interview question. Sometimes my
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brain just doesn't process certain types of problem. This was one of them. Once I made the
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connection to a weighted graph it was fine.
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