Learn Cellrun
Five ways to see the path before you draw it. Every board has exactly one answer, and these are the habits that find it without lots of backing up.
1. Join the neighbors
Every cell links to two neighbors along the path; when a cell has only two it can still reach, join both.
Think in links rather than routes. Every cell the path passes through is joined to exactly two of its side-by-side neighbors: one on the way in, one on the way out. Only the first dot and the last dot get a single link.
So when a cell that isn’t the first or last dot has only two neighbors it could still join (the others are off the board or already used up by the path), it has to join both of them. Draw those links in your head and you often get a short stretch of path for free, without knowing yet which way you’ll travel through it.
2. No branches, no traps
The path is one line: no cell gets three links, and no cut can wall off cells the line can’t come back for.
The answer is one line, so no cell can have three links: if you find yourself needing a third, an earlier choice was wrong. And the line can’t leave cells behind it with no way back: an empty cell boxed in with only one way in left has to be the finish.
Before you draw a long stretch, look at what it cuts off. If it splits the empty cells into two pockets, the path has to fill one pocket completely and then still reach the other, which it usually can’t. When you spot a pocket like that, back up to where it was made.
3. Corners commit
A corner has just two neighbors, so its stretch of path is decided before you draw anywhere near it.
A board corner has only two neighbors, so unless it holds the first or last dot it has to join both, and its little L of path is decided from the start. Mark it early. The same goes for any spot your own path turns into a corner: a free cell squeezed down to two neighbors.
Corners also commit what’s around them. Once you know the path runs along an edge into a corner and back out, the cells beside that L lose a neighbor, which can make them corners in turn. Edges often fill in this way, one forced cell after another.
4. Leave room for the tail
The last dot must be the last cell, so keep a way into it open until the very end.
The last dot isn’t just somewhere the path visits; it has to be the very last cell filled. So the area around it has to stay open until the end, with one way in left for the final stretch.
A common slip is sweeping past the last dot early and boxing it in. Before a long stretch, check that the cells next to the finish aren’t being used up, and that whatever pocket the finish sits in can still be entered from the rest of the board.
5. Work backwards
The finish is as fixed as the start: plan the last stretch from the final dot back.
The finish is fixed just like the start, so you can plan from either end. Ask how the path must arrive at the last dot: which of its neighbors could be the second-to-last cell, and how the path reaches that one from the dot before.
It works between any two dots too. If dot 5 sits in a tight spot, figure out how the path has to come into it, then make sure the route out of dot 4 can get there. Planning from the dot you’re heading to keeps you from painting yourself into a corner on the way.
Try them out: play today’s daily or an unlimited game.