Parking Lot Puzzle: Empty the Lot

The lane is everything

A car only cares about one thing: the line of squares in front of its bonnet, all the way to the edge of the lot. Cars beside it, behind it or around the corner never get in its way. So before you tap, look past the headlights. If the lane is clear, the car can go.

Nothing you do can make it worse

When a car drives away it only ever empties squares. It never blocks another car's lane. That means:

• Any car that can leave now will still be able to leave later.
• Sending a free car away is never a wrong move.
• The lot can always be emptied: every daily lot is checked by a solver before it is published.

The only way to lose time is to tap cars that cannot leave yet, or to stare at the lot without seeing which ones can.

Where to look first

1. The edge rows and columns. A car parked on the edge and facing out has an empty lane of length zero. It is always free.
2. Cars facing open lanes. A car looking down an aisle with no other cars in it is free no matter how long the aisle is.
3. Behind the car that just left. Each departure opens a strip of squares. Every car whose lane ran through that strip is a new candidate.

Planters and vans

Planters never move. A car can never leave through one, so the lots are made so that no car's only way out runs into a planter. Treat a planter as part of the wall: it shapes the aisles and creates bottlenecks around it.

Vans take three squares instead of two. They drive out the same way, but they cover more lanes, so a van parked across the middle of the lot often blocks several cars at once. Freeing a van early is a good way to open up a whole area.

Chains

On harder lots, cars wait for each other in long chains: car A is blocked by B, which is blocked by C, which is blocked by D. Follow the chain from the car you want to free until you find one with a clear lane, then clear the chain from that end. This is quicker than scanning the whole lot after every tap, and it avoids the classic mistake of tapping a car whose blocker you have not dealt with yet.

Avoiding mistakes

• Check the direction. The windscreen and the headlights are at the front of every car; the darker rear window and the tail lights at the back.
• Count the whole lane. A small car tucked into a corner of the lane is easy to miss.
• Do not tap twice in a hurry. After a bump, look at what was in the way before trying again.

A hint highlights a car that can leave right now. It counts as one mistake, so use it when you would otherwise spend a long time searching.

Difficulty and ranking

Parking Lot is ranked by time, fastest first, with fewer mistakes breaking ties. Only a member's first attempt of the day is ranked, and Undo brings the last car back into its bay without taking back a mistake.

A routine for fast clears

1. Sweep the four edges once and send every car that faces straight out.
2. Send the cars looking down empty aisles.
3. Pick the biggest knot of cars, usually around a planter or a van parked across the middle, and follow its chain to a free car.
4. After each departure, check only the cars whose lane ran through the bay that just emptied.
5. If nothing looks free for more than a few seconds, take a hint. It costs the same as one bump and ends the search at once.

With practice the lot reads almost at a glance: free cars stand out because their lanes are open, and the rest is a matter of following chains.