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PetDoku Guide: Colours Locked to One Line (Easy 688)

PetDoku The JoyPlus Soft Planning Team

You can make progress even before a pet has one certain square. If all candidates of a colour lie in one row or column, other colours cannot place a pet on that line. Easy 688 shows this restriction passing from red to orange to blue.

Rules and the starting board

Place one pet in each colour, row and column. Pets must not touch, even diagonally. Tap once for X; double-tap or hold to place a pet. Count rows from the top and columns from the left.

Easy 688 is 6x6. Yellow on the left has one cell; blue at the bottom-right has two.
Easy 688 is 6x6. Yellow on the left has one cell; blue at the bottom-right has two.

Hint 1: Place the yellow pet

Place the yellow pet at row three, column one. Mark its row, column and touching cells X.
Place the yellow pet at row three, column one. Mark its row, column and touching cells X.

Hint 2: Reserve the line for one colour

Red can only use row one, columns two or three. Either choice takes row one, so its three orange cells are X.
Red can only use row one, columns two or three. Either choice takes row one, so its three orange cells are X.
Orange is now confined to row two. The teal cell at row two, column three cannot hold a pet.
Orange is now confined to row two. The teal cell at row two, column three cannot hold a pet.

We have not chosen which of red's two cells is right. Red's row is certain even when its cell is not. That alone removes orange candidates and then fixes orange's row too. When you cannot place a pet, check which lines each colour still occupies.

Both blue cells are in column six. Mark the other-colour candidates in rows two and four of that column X.
Both blue cells are in column six. Mark the other-colour candidates in rows two and four of that column X.

Hint 3: Remove cells touched by every option

The two blue candidates in column six are in rows five and six. Either would touch column five in both rows, so those two cells are X.
The two blue candidates in column six are in rows five and six. Either would touch column five in both rows, so those two cells are X.
Green can use row four, column five or row five, column four. Removing cells ruled out by both leaves one candidate in row four.
Green can use row four, column five or row five, column four. Removing cells ruled out by both leaves one candidate in row four.

Finish the remaining single cells

Place green at row four, column five.
Place green at row four, column five.
Place orange at row two, column four.
Place orange at row two, column four.
Place red at row one, column two.
Place red at row one, column two.
Place teal at row five, column three.
Place teal at row five, column three.
The final blue pet is at row six, column six.
The final blue pet is at row six, column six.
Solved. Six pets fill the six colours, rows and columns without touching.
Solved. Six pets fill the six colours, rows and columns without touching.
TipInstead of guessing between two cells, look for their shared line and the cells both would touch. You can know an X before you know the pet.
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