4 ms·

Sum the input list, sum the list of [1...n], subtract and voila!

• robbie-c 4y ago
  Yeah, sadly you're not guaranteed some of the things that are needed for this to work. I defo believe the story that it took Knuth a day to figure this out, although when you see the answer it's pretty easy to understand.
  • deleted 4y ago
    [deleted]
  • sanedigital 4y ago
    What things? I submitted it as an answer on Leetcode and it was accepted.