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README.md

Answers

Part 1 Part 2
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--- Day 5: Doesn't He Have Intern-Elves For This? ---

Santa needs help figuring out which strings in his text file are naughty or nice.

A nice string is one with all of the following properties:

  • It contains at least three vowels (aeiou only), like aei, xazegov, or aeiouaeiouaeiou.
  • It contains at least one letter that appears twice in a row, like xx, abcdde (dd), or aabbccdd (aa, bb, cc, or dd).
  • It does not contain the strings ab, cd, pq, or xy, even if they are part of one of the other requirements.

For example:

  • ugknbfddgicrmopn is nice because it has at least three vowels (u...i...o...), a double letter (...dd...), and none of the disallowed substrings.
  • aaa is nice because it has at least three vowels and a double letter, even though the letters used by different rules overlap.
  • jchzalrnumimnmhp is naughty because it has no double letter.
  • haegwjzuvuyypxyu is naughty because it contains the string xy.
  • dvszwmarrgswjxmb is naughty because it contains only one vowel.

How many strings are nice?

--- Part Two ---

Realizing the error of his ways, Santa has switched to a better model of determining whether a string is naughty or nice. None of the old rules apply, as they are all clearly ridiculous.

Now, a nice string is one with all of the following properties:

  • It contains a pair of any two letters that appears at least twice in the string without overlapping, like xyxy (xy) or aabcdefgaa (aa), but not like aaa (aa, but it overlaps).
  • It contains at least one letter which repeats with exactly one letter between them, like xyx, abcdefeghi (efe), or even aaa.

For example:

  • qjhvhtzxzqqjkmpb is nice because is has a pair that appears twice (qj) and a letter that repeats with exactly one letter between them (zxz).
  • xxyxx is nice because it has a pair that appears twice and a letter that repeats with one between, even though the letters used by each rule overlap.
  • uurcxstgmygtbstg is naughty because it has a pair (tg) but no repeat with a single letter between them.
  • ieodomkazucvgmuy is naughty because it has a repeating letter with one between (odo), but no pair that appears twice.

How many strings are nice under these new rules?