Tap a color under the board. With a keyboard, press 1 to 6 for the colors and Backspace to undo. You get par plus 4 moves, the same as quick play in my app. Par is the shortest plan my solver found for this board, and Hint shows the color it would play next. Each color has its own symbol, so you can play without telling the colors apart.
The rules
- The flood is the area joined to the top left corner. At the start it's just the corner tile and any same colored tiles touching it.
- Each move, you pick a color. The whole flood turns that color and takes in every tile of it that touches the flood, up, down, left or right.
- The board is done when every tile is one color. The fewer moves you use, the better.
That's all of it. There's no luck after the board is dealt, so every board has a best answer, and the game is about getting close to it.
Why taking the most tiles loses
The natural way to play is to pick whichever color grows the flood the most. It feels right, and on small boards it's fine. On bigger ones it costs a lot.
I tested it. On 200 random 14x14 boards with 6 colors, one of the quick play sizes in my app, taking the most tiles every move needed 26.1 moves on average. My app's solver needed 20.3. It lost to the solver on 200 of the 200 boards, by up to 13 moves, and with the move limit of par plus 4 it would have run out of moves on 142 of them.
So the tile count is the wrong thing to watch. Here's what to watch instead.
The two numbers that matter
At any point in a game, you can work out a floor: a number of moves you can't possibly beat from here. It's the bigger of two counts.
1. Colors left
Count the colors still outside your flood. Each move gets rid of at most one of them, because you pick one color per move. If four colors are left, you need at least four more moves, however big the flood is.
2. How far the far side is
Now picture the board as patches: areas of one color that touch each other. A move reaches every patch of the chosen color that touches your flood, but never further. So a patch that's three patches away from the flood needs at least three more moves to reach.
My solver uses exactly these two counts to rank its options, and so can you. A good move lowers one of them. A move that adds plenty of tiles but leaves both counts where they were usually just moved you sideways.
A worked example
Here's a 6 by 6 board with 4 colors. I checked every possible sequence on it with an exact search: the fewest moves that flood it is 8. Taking the most tiles every move needs 10.
Going for the most tiles, you'd start with yellow, which grabs the strip along the top edge. The best first move is blue, which takes fewer tiles but heads down into the board.
| Move | Most tiles | Tiles | Colours left | Best plan | Tiles | Colours left |
|---|---|---|---|---|---|---|
| 1 | yellow | 5 | 4 | blue | 4 | 4 |
| 2 | green | 8 | 4 | green | 5 | 4 |
| 3 | red | 14 | 4 | red | 12 | 4 |
| 4 | green | 19 | 4 | blue | 16 | 4 |
| 5 | blue | 24 | 4 | green | 24 | 3 |
| 6 | red | 28 | 4 | yellow | 30 | 2 |
| 7 | blue | 31 | 3 | blue | 32 | 1 |
| 8 | yellow | 34 | 2 | red | 36 | 0 |
| 9 | green | 35 | 1 | |||
| 10 | red | 36 | 0 | |||
Look at the colors left column. Taking the most tiles, you sit at four colors from move one to move six. You keep taking tiles, but you leave a few tiles of every color behind, so you can't finish any of them off. The best plan keeps clearing colors: once it's down to three colors after move five, every move after that removes one more, and it's done in three.
Techniques, from the most useful
1. Head for the middle, then the far corner
Early on, pick colors that push the flood toward the center of the board, even when another color takes more tiles. From the middle, the flood touches more patches, so later moves take bigger bites. A flood that crawls along the edges has to come back for the middle at the end.
2. Finish colors off
When all the remaining tiles of a color touch your flood, that color can be cleared in one move, and the colors left count drops. Those moves are almost always good, and they're easy to miss when a different color offers more tiles.
3. Don't leave crumbs
Single tiles and tiny pockets cut off behind the flood are what cost you in the end. Each crumb of a different color can force one extra move. If a move would leave a lone tile walled in, check whether another order takes it with you.
4. Count the endgame
Once the flood touches every remaining patch, you need exactly one move per color left, in any order. That's the moment you can count your final score before you play it. If the count is more than the moves you have, undo back to where you still had a choice.
5. Use Hint to learn, not to finish
Hint asks the solver which color it would play from your position. Try your own move first, then press Hint and compare. When they differ, ask which count the solver's move lowers. That habit teaches more than any list of tips.
About par and the move limit
Par comes from the same beam search my app uses: it keeps the few hundred most promising floods at each move, ranked by the two counts above. I checked it against an exact search on 50 random 8x8 boards, and it found the true minimum on every one. On bigger boards an exact search takes far too long, so par is the best my solver found, not a proven minimum. If you ever beat it, the board tells you.
The move limit is par plus 4, the same as quick play in the app, so every board here can always be finished. In the app, 3 stars means par or better, 2 stars means within 2 of par, and 1 star is anything inside the limit. This board scores the same way.
Questions
Is there always a way to finish in the move limit?
Yes. The limit is par plus 4, and par is a real sequence my solver found for that exact board before you saw it.
Can a computer find the shortest answer?
On small boards, yes. I used an exact search for the 6 by 6 example above. On big boards the exact search gets slow very quickly, which is why both my app and this page use a faster search. On 8x8 boards it matched the exact answer every time I checked.
Does it matter which color I pick first?
A lot. In the example above, the first move that takes the most tiles leads to a game two moves longer than the best one.
What are the symbols for?
Each color has its own shape, the same six as in my app: circle, diamond, plus, triangle, square and two bars. You can play by shape alone if some colors look alike to you. Untick Symbols for a plain board.
Is there a version for my phone?
Yes. My Flood It app for Android has levels from 8x8 up to 22x22, a daily board that's the same for everyone, quick play, and a high contrast palette. It works offline.
Next game, before your first move, count the colors and find the far corner. You'll see the board differently.
Flood It
Levels from 8x8 to 22x22, a daily board that grows through the week, quick play and a symbol on every color.