rehrler / aoc21

advent of code 2021

aoc 21 🎄 🎅

Advent of Code repository for year '21 in Java 17

Day 1

aoc21_1.java

Hints:

• calculate differences while loading or averaging values

Day 2

aoc21_2.java

Day 3

aoc21_3.java

Hints:

• use recursion

Day 4

aoc21_4.java

Hints:

• create class for a bingo board with functions:
  • add new value at position
  • check if drawn number exists on board
  • check if any row or column is completely checked

Day 5

aoc21_5.java

Hints:

• loop over point between start and end points, not over area. would be super slow.

Day 6

aoc21_6.java

Hints:

• count fishes at stages and don't track the fish as an object. critical memory.

Day 7

aoc21_7.java

Hints:

• can be solved with brute force or optimization problem

Day 8

aoc21_8.java

Hints:

• See a combination of chars as a set and use set operations (intersect, difference)

Day 9

aoc21_9.java

Hints:

• Again, the holy power of recursion ⚡

Day 10

aoc21_10.java

Hints:

• Use a stack for opening brackets

Day 11

aoc21_11.java

Hints:

• check if you have already visited a flashing octopus 🐙

Day 12

aoc21_12.java

Hints:

• check DFS

Day 13

aoc21_13.java

Hints:

• think about what a fold means to an array and how you can optimize this.

Day 14

aoc21_14.java

Hints:

• similar to day 6

Day 15

aoc21_15.java

Hints:

• Dijkstra's algorithm

Day 16

aoc21_16.java

Day 17

aoc21_17.java

Hints:

$$y[n] = n\cdot v_y[0] + \frac{n\cdot(n-1)}{2} x[n] = \begin{cases} n\cdot v_y[0] + \frac{n\cdot(n-1)}{2} & n<v_x[0] \\\ v_y^2[0]-\frac{v_x[0]\cdot(v_x[0]-1)}{2} & n\geq v_x[0] \end{cases}$$

Day 18

aoc21_18.java

Day 19

aoc21_19.java

Hints:

• use absolute vector distance to check whether a set overlaps or not

Day 20

aoc21_20.java

Hints:

• think about what happens with the infinite pixel with your enhancing algorithm.