"Ingrid! I’m so glad to see you. But...you are a day early?"
"They predicted a blizzard in Edmonton, so I caught an earlier flight out. I hope that's OK."
"Of course, it's OK!"
Narrator: "It was not OK. I had been planning to have my Christmas decorations all put up to surprise my friend, Ingrid Deduction. I have a special present for her this year, and wanted it to be under the tree when she got here. Oh, well, plan B."
"Since you've got a surprise for me, I've got a surprise for you...we're going to pick out a Christmas tree together!"
"That sounds like fun. I saw a little tree farm, Drafty Acres, on my way from the airport. And it's a beautiful warm day. Oh let's go!"
And off we went. We wanted to beat the rush, but when we arrived...we were the only ones there, except for the attendant, Chubby. He said, "Hi folks! Thanks for coming, but I don't think we can get you a tree...all but one of them are already sold, and people are coming to pick them up tonight."
I was a bit bummed, but I really needed the tree. "That's alright, we only need one. Which one is still available?"
Chubby sighed, "Well, I don't rightly know. Our field is set up in a 10x10 grid, and to mark their tree I told folks to put their name on a flag, and to plant the flag in an empty square orthogonally adjacent to the tree they wanted. It worked out great...none of the flags were in the same or adjacent squares, not even diagonally."
I scanned the field. "Uh, Chubby...I don't see any flags."
"Yeah. Well, it's normally cold, so I borrow the flags from the golf course next door. But it's warm today, and they needed the flags back, so the grounds crew took them back overnight. Without telling me where they were."
I shook my head, but Ingrid got a twinkle in her eye, and said, "So, do you know ANYTHING about the flags?"
"Well, I did write down how many flags were in each column. But I don't see how that helps."
Ingrid broke into a dazzling grin, and said, "Oh, I do!" I knew what I was in for.
Can you figure out where all the flags were, and thus pick out which tree is available?
Online solvers: puzz.link version Note: this link takes you to edit mode, which you can use to delete the available tree in order to check your flag placement.
Thanks to Stiv for letting me borrow Ingrid for a bit of colour!
Despite the holiday dressing, this is just a Tents puzzle, with exactly one extra tree that does not have a tent/flag. Specifically, you need to place flags in the grid, with each flag "tied" to a single tree such that:
- All but one of the trees has exactly one flag tied to it. The remaining tree has no flag.
- A flag tied to a tree must be in a square orthogonally adjacent to the tree.
- Flags cannot go on squares already containing trees.
- Only one flag per square.
- No flags may be either orthogonally or diagonally adjacent.
- The numbers in each column denote the number of flags in the column.
- A flag may be adjacent (orthogonally or diagonally) to a tree to which it is not tied, if such placement does not violate other rules.
This can be solved with constraint programming, but there is a logical path through, so [no-computers] please.
I hope you enjoy!