Given a heightmap (0 to 9), for each 0 count how many 9 you can reach on paths that only ever increase height by exactly 1 at a time. Sum these values.

Given a disk layout alternating between files and empty spaces, move all files as early on the disk is possible, splitting into multiple blocks. Return a checksum on the disk.

Alternating means: 23331 would mean a 2 block file, 3 empty, a 3 block file, 3 empty, and a 1 block file.

The checksum is the sum of file_id * block_index for all occupied blocks. File IDs are assigned sequentially on initial generation.

Given a grid with either open tiles (.) or towers (anything else), for each pair of towers, there is an antinode at each of the points that is 2x as far from one tower as the other. How many antinodes are there still within the bounds of the map?

Given a result and a list of numbers, determine if any combination of addition (+) and/or multiplication (*) using all the given numbers in order can return the result. Ignore order of operations.

You are given a grid of walls (#), floors (.), and a guard (^, initially facing up/north). The guard walks forward until they run into a wall at which point they turn right. How many tiles does the guard reach before leaving the map.

The input is a list of pairs of the form a|b which defines that b must not come before a, an empty line, and then a list of values a,b,c,d.

For each line that is valid for all given a|b rules, sum the middle number of each list.

Given a grid of letters, count how many times XMAS appears in any direction (horizontally, vertically, diagonally; forward or backward).

Given a string containing (among other gibberish) commands of the form mul({A}, {B}) where {A} and {B} are integers, calculate the sum of A*Bs.

Count how many given lists of numbers are safe:

1. Sorted (either ascending or descending)
2. Strictly increasing/decreasing (no duplicate values)
3. No increase/decrease is by more than 3

Given two lists of numbers arranged in columns in a file, find the sum of differences (in sorted order).