129 lines
4.7 KiB
Plaintext
129 lines
4.7 KiB
Plaintext
|
Advent of Code
|
|||
|
|
|
|||
|
|
--- Day 7: Recursive Circus ---
|
|||
|
|
|
|||
|
|
Wandering further through the circuits of the computer, you come upon a tower
|
|||
|
|
of programs that have gotten themselves into a bit of trouble. A recursive
|
|||
|
|
algorithm has gotten out of hand, and now they're balanced precariously in a
|
|||
|
|
large tower.
|
|||
|
|
|
|||
|
|
One program at the bottom supports the entire tower. It's holding a large
|
|||
|
|
disc, and on the disc are balanced several more sub-towers. At the bottom of
|
|||
|
|
these sub-towers, standing on the bottom disc, are other programs, each
|
|||
|
|
holding their own disc, and so on. At the very tops of these
|
|||
|
|
sub-sub-sub-...-towers, many programs stand simply keeping the disc below
|
|||
|
|
them balanced but with no disc of their own.
|
|||
|
|
|
|||
|
|
You offer to help, but first you need to understand the structure of these
|
|||
|
|
towers. You ask each program to yell out their name, their weight, and (if
|
|||
|
|
they're holding a disc) the names of the programs immediately above them
|
|||
|
|
balancing on that disc. You write this information down (your puzzle input).
|
|||
|
|
Unfortunately, in their panic, they don't do this in an orderly fashion; by
|
|||
|
|
the time you're done, you're not sure which program gave which information.
|
|||
|
|
|
|||
|
|
For example, if your list is the following:
|
|||
|
|
|
|||
|
|
pbga (66)
|
|||
|
|
xhth (57)
|
|||
|
|
ebii (61)
|
|||
|
|
havc (66)
|
|||
|
|
ktlj (57)
|
|||
|
|
fwft (72) -> ktlj, cntj, xhth
|
|||
|
|
qoyq (66)
|
|||
|
|
padx (45) -> pbga, havc, qoyq
|
|||
|
|
tknk (41) -> ugml, padx, fwft
|
|||
|
|
jptl (61)
|
|||
|
|
ugml (68) -> gyxo, ebii, jptl
|
|||
|
|
gyxo (61)
|
|||
|
|
cntj (57)
|
|||
|
|
|
|||
|
|
...then you would be able to recreate the structure of the towers that looks
|
|||
|
|
like this:
|
|||
|
|
|
|||
|
|
gyxo
|
|||
|
|
/
|
|||
|
|
ugml - ebii
|
|||
|
|
/ \
|
|||
|
|
| jptl
|
|||
|
|
|
|
|||
|
|
| pbga
|
|||
|
|
/ /
|
|||
|
|
tknk --- padx - havc
|
|||
|
|
\ \
|
|||
|
|
| qoyq
|
|||
|
|
|
|
|||
|
|
| ktlj
|
|||
|
|
\ /
|
|||
|
|
fwft - cntj
|
|||
|
|
\
|
|||
|
|
xhth
|
|||
|
|
|
|||
|
|
In this example, tknk is at the bottom of the tower (the bottom program), and
|
|||
|
|
is holding up ugml, padx, and fwft. Those programs are, in turn, holding up
|
|||
|
|
other programs; in this example, none of those programs are holding up any
|
|||
|
|
other programs, and are all the tops of their own towers. (The actual tower
|
|||
|
|
balancing in front of you is much larger.)
|
|||
|
|
|
|||
|
|
Before you're ready to help them, you need to make sure your information is
|
|||
|
|
correct. What is the name of the bottom program?
|
|||
|
|
|
|||
|
|
Your puzzle answer was ___________.
|
|||
|
|
|
|||
|
|
--- Part Two ---
|
|||
|
|
|
|||
|
|
The programs explain the situation: they can't get down. Rather, they could
|
|||
|
|
get down, if they weren't expending all of their energy trying to keep the
|
|||
|
|
tower balanced. Apparently, one program has the wrong weight, and until it's
|
|||
|
|
fixed, they're stuck here.
|
|||
|
|
|
|||
|
|
For any program holding a disc, each program standing on that disc forms a
|
|||
|
|
sub-tower. Each of those sub-towers are supposed to be the same weight, or
|
|||
|
|
the disc itself isn't balanced. The weight of a tower is the sum of the
|
|||
|
|
weights of the programs in that tower.
|
|||
|
|
|
|||
|
|
In the example above, this means that for ugml's disc to be balanced, gyxo,
|
|||
|
|
ebii, and jptl must all have the same weight, and they do: 61.
|
|||
|
|
|
|||
|
|
However, for tknk to be balanced, each of the programs standing on its disc
|
|||
|
|
and all programs above it must each match. This means that the following sums
|
|||
|
|
must all be the same:
|
|||
|
|
|
|||
|
|
* ugml + (gyxo + ebii + jptl) = 68 + (61 + 61 + 61) = 251
|
|||
|
|
* padx + (pbga + havc + qoyq) = 45 + (66 + 66 + 66) = 243
|
|||
|
|
* fwft + (ktlj + cntj + xhth) = 72 + (57 + 57 + 57) = 243
|
|||
|
|
|
|||
|
|
As you can see, tknk's disc is unbalanced: ugml's stack is heavier than the
|
|||
|
|
other two. Even though the nodes above ugml are balanced, ugml itself is too
|
|||
|
|
heavy: it needs to be 8 units lighter for its stack to weigh 243 and keep the
|
|||
|
|
towers balanced. If this change were made, its weight would be 60.
|
|||
|
|
|
|||
|
|
Given that exactly one program is the wrong weight, what would its weight
|
|||
|
|
need to be to balance the entire tower?
|
|||
|
|
|
|||
|
|
Your puzzle answer was ______.
|
|||
|
|
|
|||
|
|
Both parts of this puzzle are complete! They provide two gold stars: **
|
|||
|
|
|
|||
|
|
At this point, you should return to your advent calendar and try another
|
|||
|
|
puzzle.
|
|||
|
|
|
|||
|
|
If you still want to see it, you can get your puzzle input.
|
|||
|
|
|
|||
|
|
References
|
|||
|
|
|
|||
|
|
Visible links
|
|||
|
|
. http://adventofcode.com/
|
|||
|
|
. http://adventofcode.com/2017/about
|
|||
|
|
. http://adventofcode.com/2017/support
|
|||
|
|
. http://adventofcode.com/2017/events
|
|||
|
|
. http://adventofcode.com/2017/settings
|
|||
|
|
. http://adventofcode.com/2017/auth/logout
|
|||
|
|
. http://adventofcode.com/2017
|
|||
|
|
. http://adventofcode.com/2017
|
|||
|
|
. http://adventofcode.com/2017/leaderboard
|
|||
|
|
. http://adventofcode.com/2017/stats
|
|||
|
|
. http://adventofcode.com/2017/sponsors
|
|||
|
|
. http://adventofcode.com/2017/sponsors
|
|||
|
|
. http://adventofcode.com/2017
|
|||
|
|
. http://adventofcode.com/2017/day/7/input
|