# A Teacher looks at Advent of Code 2020 - days 9 and 10

As we get closer to the end of the semester and time becomes scarcer I'm wondering how many more I'll finish. Barely had time to do days 9 and 10.

Not much to say about day 9 Part one was basically a rehash of day 1 part 1 but with a sliding widow. Part 2? I just brute force tried all the subranges. I meant to go back to try to improve the solution but didn't have a chance.

Clojure code can be found here.

Day 10 was more interesting.

I misread part 1 but ultimately, my solution was to:

2. sort it

3. prepend a 0

4. append an additional value of the max + 3 to the end

Then loop through comparing adjacent values and keep track of the differences which can be 1, 2, or 3.

``````data=[ int(x) for x in open("../data/sample10-1.dat").readlines()]
data.sort()
data.insert(0,0)
data.append(max(data)+3)

j=[0,0,0]
for i in range(len(data)-1):
diff=data[i+1]-data[i]
j[diff-1]=j[diff-1]+1

print(j)``````

Part 2 was where the fun starts - how many combinations of adapters will take you from the start to the end.

For example, if we had adapters with voltages 1,4,5,6 given the problem constraints that you can only connect an adapter to another with a "joltage" of up to three less, you can only use the 1 voltage 1 way (coming from the source of 0).

4 can connect to only 1 so it can only be used 1 way.

5 can connect to 4 so it too can only be used 1 way.

6 is a change, it can connect to 5 or 4 so you could chain either 6–>5–>4–>1 or 6–>4–>1 so you can get to 6 two ways.

If a student knows recursion and recursive search it's easy enough to code something that tries all the paths but it's going to get very slow very fast.

With a couple of insights though this can lead to a nice dynamic programming type solution.

First thing to notice is that, similar to day 7, instead of looking at how many adapters a lower joltage adapter can lead to we can look at how many lower joltage adapters a given adapter could have come from.

To do this, we can look at the adapters as a graph. We can build a dictionary where the keys are the adapter joltages and the entries are the lower joltage adapters it can connect with:

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 `````` ``````def build_reverse_map(data): graph={} data = data[::-1] for i in range(len(data)): current = data[i] j=i+1; while (j

Line 3 reverses the list then for each item in the list, lines 6 through 8 find the adapters it can connect with.

The next insight is that instead of calculating all the possibilities we can build them a step at a time.

Consider the first voltage from our above example of 1,4,5,6.

1 – it can only go to 0 so 0 is its only "neighbor" in the graph. We can only get there 1 way so we can store 1 in ways.

4 – same deal - its only neighbor is 1. You can only get to 1 one way so you can only get to 4 1 way - store it in ways

5 – same deal, ways = 1.

6 - now six is different. It has 2 neighbors - 5 and 4. You can get to 5 one way and 4 one way so we can get to 6 two ways (the sum of the ways to get to each of it's neighbors). Store that in ways

Go through all the nodes and then ways[the last node] will have your answer:

 `````` 1 2 3 4 5 6 7 8 9 10 `````` ``````ways={} for d in data: ways[d]=1 for d in data[1:]: neighbors = rmap[d] sum = 0; for n in neighbors: sum = sum + ways[n] ways[d]=sum``````

Relatively straightforward and lightning fast. Dynamic programming can be really hard to teach but I think this problem might be a good one to do with an advanced data structures class.

For comparison, you can find the clojure code here.

Fun problem today. Looking forward to tomorrow.