The 2021 problem theme is undersea adventures in Santa’s submarine, looking for the keys to Santa’s sled, which have been accidentally deep-sixed from Santa’s ship.
- AoC '21, Day 1
- AoC '21, Day 2
- AoC '21, Day 3
- AoC '21, Day 4
- AoC '21, Day 5
- AoC '21, Day 6
- AoC '21, Day 7
- AoC '21, Day 8
- AoC '21, Day 9
- AoC '21, Day 10
- AoC '21, Day 11
- AoC '21, Day 12
- AoC '21, Day 13
- AoC '21, Day 14
- AoC '21, Day 15
- AoC '21, Day 16
- AoC '21, Day 17
- AoC '21, Day 18