2025 Advent of Code Summary

Note: the number of problems has been changed from 25 to 12 this year.

Day   -Part 1-   -Part 2-
  5   00:03:50   00:06:40
  4   00:04:18   00:08:48
  3   00:03:38   00:09:51
  2   00:05:01   00:23:55
  1   00:02:54   00:12:32

Day 5

  1. Brute-force.
  2. Merge intervals.

Day 4

  1. Simulation.
  2. Simulation, but multiple rounds.

Day 3

  1. Brute-force.
  2. Treat each row as: “remove some digits so the remaining 12 are as big as possible from left to right”. Hint: use monotonic stack.

Day 2

  1. Brute-force. For each range (l, r), check all integers from 1 up to half the number of digits in r, inclusive.
  2. Similar idea, but remember to handle duplicates. For example, 1111 can be represented as 1 repeated four times or 11 repeated two times.

Day 1

  1. Modular arithmetic.
  2. Brute-force.

Last edited: Dec 04, 2025